Adding upstream version 1:115.7.0.upstream/1%115.7.0upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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diff --git a/docs/code-quality/coding-style/coding_style_cpp.rst b/docs/code-quality/coding-style/coding_style_cpp.rst
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+================
+C++ Coding style
+================
+
+
+This document attempts to explain the basic styles and patterns used in
+the Mozilla codebase. New code should try to conform to these standards,
+so it is as easy to maintain as existing code. There are exceptions, but
+it's still important to know the rules!
+
+This article is particularly for those new to the Mozilla codebase, and
+in the process of getting their code reviewed. Before requesting a
+review, please read over this document, making sure that your code
+conforms to recommendations.
+
+.. container:: blockIndicator warning
+
+   The Firefox code base adopts parts of the `Google Coding style for C++
+   code <https://google.github.io/styleguide/cppguide.html>`__, but not all of its rules.
+   A few rules are followed across the code base, others are intended to be
+   followed in new or significantly revised code. We may extend this list in the
+   future, when we evaluate the Google Coding Style for C++ Code further and/or update
+   our coding practices. However, the plan is not to adopt all rules of the Google Coding
+   Style for C++ Code. Some rules are explicitly unlikely to be adopted at any time.
+
+   Followed across the code base:
+
+   - `Formatting <https://google.github.io/styleguide/cppguide.html#Formatting>`__,
+     except for subsections noted here otherwise
+   - `Implicit Conversions <https://google.github.io/styleguide/cppguide.html#Implicit_Conversions>`__,
+     which is enforced by a custom clang-plugin check, unless explicitly overridden using
+     ``MOZ_IMPLICIT``
+
+   Followed in new/significantly revised code:
+
+   - `Include guards <https://google.github.io/styleguide/cppguide.html#The__define_Guard>`__
+
+   Unlikely to be ever adopted:
+
+   - `Forward declarations <https://google.github.io/styleguide/cppguide.html#Forward_Declarations>`__
+   - `Formatting/Conditionals <https://google.github.io/styleguide/cppguide.html#Conditionals>`__
+     w.r.t. curly braces around inner statements, we require them in all cases where the
+     Google style allows to leave them out for single-line conditional statements
+
+   This list reflects the state of the Google Google Coding Style for C++ Code as of
+   2020-07-17. It may become invalid when the Google modifies its Coding Style.
+
+
+Formatting code
+---------------
+
+Formatting is done automatically via clang-format, and controlled via in-tree
+configuration files. See :ref:`Formatting C++ Code With clang-format`
+for more information.
+
+Unix-style linebreaks (``\n``), not Windows-style (``\r\n``). You can
+convert patches, with DOS newlines to Unix via the ``dos2unix`` utility,
+or your favorite text editor.
+
+Static analysis
+---------------
+
+Several of the rules in the Google C++ coding styles and the additions mentioned below
+can be checked via clang-tidy (some rules are from the upstream clang-tidy, some are
+provided via a mozilla-specific plugin). Some of these checks also allow fixes to
+be automatically applied.
+
+``mach static-analysis`` provides a convenient way to run these checks. For example,
+for the check called ``google-readability-braces-around-statements``, you can run:
+
+.. code-block:: shell
+
+   ./mach static-analysis check --checks="-*,google-readability-braces-around-statements" --fix <file>
+
+It may be necessary to reformat the files after automatically applying fixes, see
+:ref:`Formatting C++ Code With clang-format`.
+
+Additional rules
+----------------
+
+*The norms in this section should be followed for new code. For existing code,
+use the prevailing style in a file or module, ask the owner if you are
+in another team's codebase or it's not clear what style to use.*
+
+
+
+
+Control structures
+~~~~~~~~~~~~~~~~~~
+
+Always brace controlled statements, even a single-line consequent of
+``if else else``. This is redundant, typically, but it avoids dangling
+else bugs, so it's safer at scale than fine-tuning.
+
+Examples:
+
+.. code-block:: cpp
+
+   if (...) {
+   } else if (...) {
+   } else {
+   }
+
+   while (...) {
+   }
+
+   do {
+   } while (...);
+
+   for (...; ...; ...) {
+   }
+
+   switch (...) {
+     case 1: {
+       // When you need to declare a variable in a switch, put the block in braces.
+       int var;
+       break;
+     }
+     case 2:
+       ...
+       break;
+     default:
+       break;
+   }
+
+``else`` should only ever be followed by ``{`` or ``if``; i.e., other
+control keywords are not allowed and should be placed inside braces.
+
+.. note::
+
+   For this rule, clang-tidy provides the ``google-readability-braces-around-statements``
+   check with autofixes.
+
+
+C++ namespaces
+~~~~~~~~~~~~~~
+
+Mozilla project C++ declarations should be in the ``mozilla``
+namespace. Modules should avoid adding nested namespaces under
+``mozilla``, unless they are meant to contain names which have a high
+probability of colliding with other names in the code base. For example,
+``Point``, ``Path``, etc. Such symbols can be put under
+module-specific namespaces, under ``mozilla``, with short
+all-lowercase names. Other global namespaces besides ``mozilla`` are
+not allowed.
+
+No ``using`` directives are allowed in header files, except inside class
+definitions or functions. (We don't want to pollute the global scope of
+compilation units that use the header file.)
+
+.. note::
+
+   For parts of this rule, clang-tidy provides the ``google-global-names-in-headers``
+   check. It only detects ``using namespace`` directives in the global namespace.
+
+
+``using namespace ...;`` is only allowed in ``.cpp`` files after all
+``#include``\ s. Prefer to wrap code in ``namespace ... { ... };``
+instead, if possible. ``using namespace ...;``\ should always specify
+the fully qualified namespace. That is, to use ``Foo::Bar`` do not
+write ``using namespace Foo; using namespace Bar;``, write
+``using namespace Foo::Bar;``
+
+Use nested namespaces (ex: ``namespace mozilla::widget {``
+
+.. note::
+
+   clang-tidy provides the ``modernize-concat-nested-namespaces``
+   check with autofixes.
+
+
+Anonymous namespaces
+~~~~~~~~~~~~~~~~~~~~
+
+We prefer using ``static``, instead of anonymous C++ namespaces. This may
+change once there is better debugger support (especially on Windows) for
+placing breakpoints, etc. on code in anonymous namespaces. You may still
+use anonymous namespaces for things that can't be hidden with ``static``,
+such as types, or certain objects which need to be passed to template
+functions.
+
+
+C++ classes
+~~~~~~~~~~~~
+
+.. code-block:: cpp
+
+   namespace mozilla {
+
+   class MyClass : public A
+   {
+     ...
+   };
+
+   class MyClass
+     : public X
+     , public Y
+   {
+   public:
+     MyClass(int aVar, int aVar2)
+       : mVar(aVar)
+       , mVar2(aVar2)
+     {
+        ...
+     }
+
+     // Special member functions, like constructors, that have default bodies
+     // should use '= default' annotation instead.
+     MyClass() = default;
+
+     // Unless it's a copy or move constructor or you have a specific reason to allow
+     // implicit conversions, mark all single-argument constructors explicit.
+     explicit MyClass(OtherClass aArg)
+     {
+       ...
+     }
+
+     // This constructor can also take a single argument, so it also needs to be marked
+     // explicit.
+     explicit MyClass(OtherClass aArg, AnotherClass aArg2 = AnotherClass())
+     {
+       ...
+     }
+
+     int LargerFunction()
+     {
+       ...
+       ...
+     }
+
+   private:
+     int mVar;
+   };
+
+   } // namespace mozilla
+
+Define classes using the style given above.
+
+.. note::
+
+   For the rule on ``= default``, clang-tidy provides the ``modernize-use-default``
+   check with autofixes.
+
+   For the rule on explicit constructors and conversion operators, clang-tidy
+   provides the ``mozilla-implicit-constructor`` check.
+
+Existing classes in the global namespace are named with a short prefix
+(For example, ``ns``) as a pseudo-namespace.
+
+
+Methods and functions
+~~~~~~~~~~~~~~~~~~~~~
+
+
+C/C++
+^^^^^
+
+In C/C++, method names should use ``UpperCamelCase``.
+
+Getters that never fail, and never return null, are named ``Foo()``,
+while all other getters use ``GetFoo()``. Getters can return an object
+value, via a ``Foo** aResult`` outparam (typical for an XPCOM getter),
+or as an ``already_AddRefed<Foo>`` (typical for a WebIDL getter,
+possibly with an ``ErrorResult& rv`` parameter), or occasionally as a
+``Foo*`` (typical for an internal getter for an object with a known
+lifetime). See `the bug 223255 <https://bugzilla.mozilla.org/show_bug.cgi?id=223255>`_
+for more information.
+
+XPCOM getters always return primitive values via an outparam, while
+other getters normally use a return value.
+
+Method declarations must use, at most, one of the following keywords:
+``virtual``, ``override``, or ``final``. Use ``virtual`` to declare
+virtual methods, which do not override a base class method with the same
+signature. Use ``override`` to declare virtual methods which do
+override a base class method, with the same signature, but can be
+further overridden in derived classes. Use ``final`` to declare virtual
+methods which do override a base class method, with the same signature,
+but can NOT be further overridden in the derived classes. This should
+help the person reading the code fully understand what the declaration
+is doing, without needing to further examine base classes.
+
+.. note::
+
+   For the rule on ``virtual/override/final``, clang-tidy provides the
+   ``modernize-use-override`` check with autofixes.
+
+
+Operators
+~~~~~~~~~
+
+The unary keyword operator ``sizeof``, should have its operand parenthesized
+even if it is an expression; e.g. ``int8_t arr[64]; memset(arr, 42, sizeof(arr));``.
+
+
+Literals
+~~~~~~~~
+
+Use ``\uXXXX`` unicode escapes for non-ASCII characters. The character
+set for XUL, script, and properties files is UTF-8, which is not easily
+readable.
+
+
+Prefixes
+~~~~~~~~
+
+Follow these naming prefix conventions:
+
+
+Variable prefixes
+^^^^^^^^^^^^^^^^^
+
+-  k=constant (e.g. ``kNC_child``). Not all code uses this style; some
+   uses ``ALL_CAPS`` for constants.
+-  g=global (e.g. ``gPrefService``)
+-  a=argument (e.g. ``aCount``)
+-  C++ Specific Prefixes
+
+   -  s=static member (e.g. ``sPrefChecked``)
+   -  m=member (e.g. ``mLength``)
+   -  e=enum variants (e.g. ``enum Foo { eBar, eBaz }``). Enum classes
+      should use ``CamelCase`` instead (e.g.
+      ``enum class Foo { Bar, Baz }``).
+
+
+Global functions/macros/etc
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+-  Macros begin with ``MOZ_``, and are all caps (e.g.
+   ``MOZ_WOW_GOODNESS``). Note that older code uses the ``NS_`` prefix;
+   while these aren't being changed, you should only use ``MOZ_`` for
+   new macros. The only exception is if you're creating a new macro,
+   which is part of a set of related macros still using the old ``NS_``
+   prefix. Then you should be consistent with the existing macros.
+
+
+Error Variables
+^^^^^^^^^^^^^^^
+
+-  Local variables that are assigned ``nsresult`` result codes should be named ``rv``
+   (i.e., e.g., not ``res``, not ``result``, not ``foo``). `rv` should not be
+   used for bool or other result types.
+-  Local variables that are assigned ``bool`` result codes should be named `ok`.
+
+
+C/C++ practices
+---------------
+
+-  **Have you checked for compiler warnings?** Warnings often point to
+   real bugs. `Many of them <https://searchfox.org/mozilla-central/source/build/moz.configure/warnings.configure>`__
+   are enabled by default in the build system.
+-  In C++ code, use ``nullptr`` for pointers. In C code, using ``NULL``
+   or ``0`` is allowed.
+
+.. note::
+
+   For the C++ rule, clang-tidy provides the ``modernize-use-nullptr`` check
+   with autofixes.
+
+-  Don't use ``PRBool`` and ``PRPackedBool`` in C++, use ``bool``
+   instead.
+-  For checking if a ``std`` container has no items, don't use
+   ``size()``, instead use ``empty()``.
+-  When testing a pointer, use ``(!myPtr)`` or ``(myPtr)``;
+   don't use ``myPtr != nullptr`` or ``myPtr == nullptr``.
+-  Do not compare ``x == true`` or ``x == false``. Use ``(x)`` or
+   ``(!x)`` instead. ``if (x == true)`` may have semantics different from
+   ``if (x)``!
+
+.. note::
+
+   clang-tidy provides the ``readability-simplify-boolean-expr`` check
+   with autofixes that checks for these and some other boolean expressions
+   that can be simplified.
+
+-  In general, initialize variables with ``nsFoo aFoo = bFoo,`` and not
+   ``nsFoo aFoo(bFoo)``.
+
+   -  For constructors, initialize member variables with : ``nsFoo
+      aFoo(bFoo)`` syntax.
+
+-  To avoid warnings created by variables used only in debug builds, use
+   the
+   `DebugOnly<T> <https://developer.mozilla.org/docs/Mozilla/Debugging/DebugOnly%3CT%3E>`__
+   helper when declaring them.
+-  You should `use the static preference
+   API <https://firefox-source-docs.mozilla.org/modules/libpref/index.html>`__ for
+   working with preferences.
+-  One-argument constructors, that are not copy or move constructors,
+   should generally be marked explicit. Exceptions should be annotated
+   with ``MOZ_IMPLICIT``.
+-  Use ``char32_t`` as the return type or argument type of a method that
+   returns or takes as argument a single Unicode scalar value. (Don't
+   use UTF-32 strings, though.)
+-  Prefer unsigned types for semantically-non-negative integer values.
+-  When operating on integers that could overflow, use ``CheckedInt``.
+-  Avoid the usage of ``typedef``, instead, please use ``using`` instead.
+
+.. note::
+
+   For parts of this rule, clang-tidy provides the ``modernize-use-using``
+   check with autofixes.
+
+
+Header files
+------------
+
+Since the Firefox code base is huge and uses a monolithic build, it is
+of utmost importance for keeping build times reasonable to limit the
+number of included files in each translation unit to the required minimum.
+Exported header files need particular attention in this regard, since their
+included files propagate, and many of them are directly or indirectly
+included in a large number of translation units.
+
+-  Include guards are named per the Google coding style (i.e. upper snake
+   case with a single trailing underscore). They should not include a
+   leading ``MOZ_`` or ``MOZILLA_``. For example, ``dom/media/foo.h``
+   would use the guard ``DOM_MEDIA_FOO_H_``.
+-  Forward-declare classes in your header files, instead of including
+   them, whenever possible. For example, if you have an interface with a
+   ``void DoSomething(nsIContent* aContent)`` function, forward-declare
+   with ``class nsIContent;`` instead of ``#include "nsIContent.h"``.
+   If a "forwarding header" is provided for a type, include that instead of
+   putting the literal forward declaration(s) in your header file. E.g. for
+   some JavaScript types, there is ``js/TypeDecls.h``, for the string types
+   there is ``StringFwd.h``. One reason for this is that this allows
+   changing a type to a type alias by only changing the forwarding header.
+   The following uses of a type can be done with a forward declaration only:
+
+   -  Parameter or return type in a function declaration
+   -  Member/local variable pointer or reference type
+   -  Use as a template argument (not in all cases) in a member/local variable type
+   -  Defining a type alias
+
+   The following uses of a type require a full definition:
+
+   -  Base class
+   -  Member/local variable type
+   -  Use with delete or new
+   -  Use as a template argument (not in all cases)
+   -  Any uses of non-scoped enum types
+   -  Enum values of a scoped enum type
+
+   Use as a template argument is somewhat tricky. It depends on how the
+   template uses the type. E.g. ``mozilla::Maybe<T>`` and ``AutoTArray<T>``
+   always require a full definition of ``T`` because the size of the
+   template instance depends on the size of ``T``. ``RefPtr<T>`` and
+   ``UniquePtr<T>`` don't require a full definition (because their
+   pointer member always has the same size), but their destructor
+   requires a full definition. If you encounter a template that cannot
+   be instantiated with a forward declaration only, but it seems
+   it should be possible, please file a bug (if it doesn't exist yet).
+
+   Therefore, also consider the following guidelines to allow using forward
+   declarations as widely as possible.
+-  Inline function bodies in header files often pull in a lot of additional
+   dependencies. Be mindful when adding or extending inline function bodies,
+   and consider moving the function body to the cpp file or to a separate
+   header file that is not included everywhere. Bug 1677553 intends to provide
+   a more specific guideline on this.
+-  Consider the use of the `Pimpl idiom <https://en.cppreference.com/w/cpp/language/pimpl>`__,
+   i.e. hide the actual implementation in a separate ``Impl`` class that is
+   defined in the implementation file and only expose a ``class Impl;`` forward
+   declaration and ``UniquePtr<Impl>`` member in the header file.
+-  Do not use non-scoped enum types. These cannot be forward-declared. Use
+   scoped enum types instead, and forward declare them when possible.
+-  Avoid nested types that need to be referenced from outside the class.
+   These cannot be forward declared. Place them in a namespace instead, maybe
+   in an extra inner namespace, and forward declare them where possible.
+-  Avoid mixing declarations with different sets of dependencies in a single
+   header file. This is generally advisable, but even more so when some of these
+   declarations are used by a subset of the translation units that include the
+   combined header file only. Consider such a badly mixed header file like:
+
+   .. code-block:: cpp
+
+      /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+      /* vim: set ts=8 sts=2 et sw=2 tw=80: */
+      /* This Source Code Form is subject to the terms of the Mozilla Public
+      * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+      * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+      #ifndef BAD_MIXED_FILE_H_
+      #define BAD_MIXED_FILE_H_
+
+      // Only this include is needed for the function declaration below.
+      #include "nsCOMPtr.h"
+
+      // These includes are only needed for the class definition.
+      #include "nsIFile.h"
+      #include "mozilla/ComplexBaseClass.h"
+
+      namespace mozilla {
+
+      class WrappedFile : public nsIFile, ComplexBaseClass {
+      // ... class definition left out for clarity
+      };
+
+      // Assuming that most translation units that include this file only call
+      // the function, but don't need the class definition, this should be in a
+      // header file on its own in order to avoid pulling in the other
+      // dependencies everywhere.
+      nsCOMPtr<nsIFile> CreateDefaultWrappedFile(nsCOMPtr<nsIFile>&& aFileToWrap);
+
+      } // namespace mozilla
+
+      #endif // BAD_MIXED_FILE_H_
+
+
+An example header file based on these rules (with some extra comments):
+
+.. code-block:: cpp
+
+   /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+   /* vim: set ts=8 sts=2 et sw=2 tw=80: */
+   /* This Source Code Form is subject to the terms of the Mozilla Public
+   * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+   * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+   #ifndef DOM_BASE_FOO_H_
+   #define DOM_BASE_FOO_H_
+
+   // Include guards should come at the very beginning and always use exactly
+   // the style above. Otherwise, compiler optimizations that avoid rescanning
+   // repeatedly included headers might not hit and cause excessive compile
+   // times.
+
+   #include <cstdint>
+   #include "nsCOMPtr.h"  // This is needed because we have a nsCOMPtr<T> data member.
+
+   class nsIFile;  // Used as a template argument only.
+   enum class nsresult : uint32_t; // Used as a parameter type only.
+   template <class T>
+   class RefPtr;   // Used as a return type only.
+
+   namespace mozilla::dom {
+
+   class Document; // Used as a template argument only.
+
+   // Scoped enum, not as a nested type, so it can be
+   // forward-declared elsewhere.
+   enum class FooKind { Small, Big };
+
+   class Foo {
+   public:
+     // Do not put the implementation in the header file, it would
+     // require including nsIFile.h
+     Foo(nsCOMPtr<nsIFile> aFile, FooKind aFooKind);
+
+     RefPtr<Document> CreateDocument();
+
+     void SetResult(nsresult aResult);
+
+     // Even though we will default this destructor, do this in the
+     // implementation file since we would otherwise need to include
+     // nsIFile.h in the header.
+     ~Foo();
+
+   private:
+     nsCOMPtr<nsIFile> mFile;
+   };
+
+   } // namespace mozilla::dom
+
+   #endif // DOM_BASE_FOO_H_
+
+
+Corresponding implementation file:
+
+.. code-block:: cpp
+
+   /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+   /* vim: set ts=8 sts=2 et sw=2 tw=80: */
+   /* This Source Code Form is subject to the terms of the Mozilla Public
+   * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+   * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+   #include "mozilla/dom/Foo.h"  // corresponding header
+
+   #include "mozilla/Assertions.h"  // Needed for MOZ_ASSERT.
+   #include "mozilla/dom/Document.h" // Needed because we construct a Document.
+   #include "nsError.h"  // Needed because we use NS_OK aka nsresult::NS_OK.
+   #include "nsIFile.h"  // This is needed because our destructor indirectly calls delete nsIFile in a template instance.
+
+   namespace mozilla::dom {
+
+   // Do not put the implementation in the header file, it would
+   // require including nsIFile.h
+   Foo::Foo(nsCOMPtr<nsIFile> aFile, FooKind aFooKind)
+    : mFile{std::move(aFile)} {
+   }
+
+   RefPtr<Document> Foo::CreateDocument() {
+     return MakeRefPtr<Document>();
+   }
+
+   void Foo::SetResult(nsresult aResult) {
+      MOZ_ASSERT(aResult != NS_OK);
+
+      // do something with aResult
+   }
+
+   // Even though we default this destructor, do this in the
+   // implementation file since we would otherwise need to include
+   // nsIFile.h in the header.
+   Foo::~Foo() = default;
+
+   } // namespace mozilla::dom
+
+
+Include directives
+------------------
+
+- Ordering:
+
+  - In an implementation file (cpp file), the very first include directive
+    should include the corresponding header file, followed by a blank line.
+  - Any conditional includes (depending on some ``#ifdef`` or similar) follow
+    after non-conditional includes. Don't mix them in.
+  - Don't place comments between non-conditional includes.
+
+  Bug 1679522 addresses automating the ordering via clang-format, which
+  is going to enforce some stricter rules. Expect the includes to be reordered.
+  If you include third-party headers that are not self-contained, and therefore
+  need to be included in a particular order, enclose those (and only those)
+  between ``// clang-format off`` and ``// clang-format on``. This should not be
+  done for Mozilla headers, which should rather be made self-contained if they
+  are not.
+
+- Brackets vs. quotes: C/C++ standard library headers are included using
+  brackets (e.g. ``#include <cstdint>``), all other include directives use
+  (double) quotes (e.g. ``#include "mozilla/dom/Document.h``).
+- Exported headers should always be included from their exported path, not
+  from their source path in the tree, even if available locally. E.g. always
+  do ``#include "mozilla/Vector.h"``, not ``#include "Vector.h"``, even
+  from within `mfbt`.
+- Generally, you should include exactly those headers that are needed, not
+  more and not less. Unfortunately this is not easy to see. Maybe C++20
+  modules will bring improvements to this, but it will take a long time
+  to be adopted.
+- The basic rule is that if you literally use a symbol in your file that
+  is declared in a header A.h, include that header. In particular in header
+  files, check if a forward declaration or including a forwarding header is
+  sufficient, see section :ref:`Header files`.
+
+  There are cases where this basic rule is not sufficient. Some cases where
+  you need to include additional headers are:
+
+  - You reference a member of a type that is not literally mentioned in your
+    code, but, e.g. is the return type of a function you are calling.
+
+  There are also cases where the basic rule leads to redundant includes. Note
+  that "redundant" here does not refer to "accidentally redundant" headers,
+  e.g. at the time of writing ``mozilla/dom/BodyUtil.h`` includes
+  ``mozilla/dom/FormData.h``, but it doesn't need to (it only needs a forward
+  declaration), so including ``mozilla/dom/FormData.h`` is "accidentally
+  redundant" when including ``mozilla/dom/BodyUtil.h``. The includes of
+  ``mozilla/dom/BodyUtil.h`` might change at any time, so if a file that
+  includes ``mozilla/dom/BodyUtil.h`` needs a full definition of
+  ``mozilla::dom::FormData``, it should includes ``mozilla/dom/FormData.h``
+  itself. In fact, these "accidentally redundant" headers MUST be included.
+  Relying on accidentally redundant includes makes any change to a header
+  file extremely hard, in particular when considering that the set of
+  accidentally redundant includes differs between platforms.
+  But some cases in fact are non-accidentally redundant, and these can and
+  typically should not be repeated:
+
+  - The includes of the header file do not need to be repeated in its
+    corresponding implementation file. Rationale: the implementation file and
+    its corresponding header file are tightly coupled per se.
+
+  Macros are a special case. Generally, the literal rule also applies here,
+  i.e. if the macro definition references a symbol, the file containing the
+  macro definition should include the header defining the symbol. E.g.
+  ``NS_IMPL_CYCLE_COLLECTING_NATIVE_RELEASE`` defined in ``nsISupportsImpl.h``
+  makes use of ``MOZ_ASSERT`` defined in ``mozilla/Assertions.h``, so
+  ``nsISupportsImpl.h`` includes ``mozilla/Assertions.h``. However, this
+  requires human judgment of what is intended, since technically only the
+  invocations of the macro reference a symbol (and that's how
+  include-what-you-use handles this). It might depend on the
+  context or parameters which symbol is actually referenced, and sometimes
+  this is on purpose. In these cases, the user of the macro needs to include
+  the required header(s).
+
+
+
+COM and pointers
+----------------
+
+-  Use ``nsCOMPtr<>``
+   If you don't know how to use it, start looking in the code for
+   examples. The general rule, is that the very act of typing
+   ``NS_RELEASE`` should be a signal to you to question your code:
+   "Should I be using ``nsCOMPtr`` here?". Generally the only valid use
+   of ``NS_RELEASE`` is when you are storing refcounted pointers in a
+   long-lived datastructure.
+-  Declare new XPCOM interfaces using :doc:`XPIDL </xpcom/xpidl>`, so they
+   will be scriptable.
+-  Use :doc:`nsCOMPtr </xpcom/refptr>` for strong references, and
+   ``nsWeakPtr`` for weak references.
+-  Don't use ``QueryInterface`` directly. Use ``CallQueryInterface`` or
+   ``do_QueryInterface`` instead.
+-  Use :ref:`Contract IDs <contract_ids>`,
+   instead of CIDs with ``do_CreateInstance``/``do_GetService``.
+-  Use pointers, instead of references for function out parameters, even
+   for primitive types.
+
+
+IDL
+---
+
+
+Use leading-lowercase, or "interCaps"
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+When defining a method or attribute in IDL, the first letter should be
+lowercase, and each following word should be capitalized. For example:
+
+.. code-block:: cpp
+
+   long updateStatusBar();
+
+
+Use attributes wherever possible
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Whenever you are retrieving or setting a single value, without any
+context, you should use attributes. Don't use two methods when you could
+use an attribute. Using attributes logically connects the getting and
+setting of a value, and makes scripted code look cleaner.
+
+This example has too many methods:
+
+.. code-block:: cpp
+
+   interface nsIFoo : nsISupports
+   {
+       long getLength();
+       void setLength(in long length);
+       long getColor();
+   };
+
+The code below will generate the exact same C++ signature, but is more
+script-friendly.
+
+.. code-block:: cpp
+
+   interface nsIFoo : nsISupports
+   {
+       attribute long length;
+       readonly attribute long color;
+   };
+
+
+Use Java-style constants
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+When defining scriptable constants in IDL, the name should be all
+uppercase, with underscores between words:
+
+.. code-block:: cpp
+
+   const long ERROR_UNDEFINED_VARIABLE = 1;
+
+
+See also
+~~~~~~~~
+
+For details on interface development, as well as more detailed style
+guides, see the `Interface development
+guide <https://developer.mozilla.org/docs/Mozilla/Developer_guide/Interface_development_guide>`__.
+
+
+Error handling
+--------------
+
+
+Check for errors early and often
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Every time you make a call into an XPCOM function, you should check for
+an error condition. You need to do this even if you know that call will
+never fail. Why?
+
+-  Someone may change the callee in the future to return a failure
+   condition.
+-  The object in question may live on another thread, another process,
+   or possibly even another machine. The proxy could have failed to make
+   your call in the first place.
+
+Also, when you make a new function which is failable (i.e. it will
+return a ``nsresult`` or a ``bool`` that may indicate an error), you should
+explicitly mark the return value should always be checked. For example:
+
+::
+
+   // for IDL.
+   [must_use] nsISupports
+   create();
+
+   // for C++, add this in *declaration*, do not add it again in implementation.
+   [[nodiscard]] nsresult
+   DoSomething();
+
+There are some exceptions:
+
+-  Predicates or getters, which return ``bool`` or ``nsresult``.
+-  IPC method implementation (For example, ``bool RecvSomeMessage()``).
+-  Most callers will check the output parameter, see below.
+
+.. code-block:: cpp
+
+   nsresult
+   SomeMap::GetValue(const nsString& key, nsString& value);
+
+If most callers need to check the output value first, then adding
+``[[nodiscard]]`` might be too verbose. In this case, change the return value
+to void might be a reasonable choice.
+
+There is also a static analysis attribute ``[[nodiscard]]``, which can
+be added to class declarations, to ensure that those declarations are
+always used when they are returned.
+
+
+Use the NS_WARN_IF macro when errors are unexpected.
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The ``NS_WARN_IF`` macro can be used to issue a console warning, in debug
+builds if the condition fails. This should only be used when the failure
+is unexpected and cannot be caused by normal web content.
+
+If you are writing code which wants to issue warnings when methods fail,
+please either use ``NS_WARNING`` directly, or use the new ``NS_WARN_IF`` macro.
+
+.. code-block:: cpp
+
+   if (NS_WARN_IF(somethingthatshouldbefalse)) {
+     return NS_ERROR_INVALID_ARG;
+   }
+
+   if (NS_WARN_IF(NS_FAILED(rv))) {
+     return rv;
+   }
+
+Previously, the ``NS_ENSURE_*`` macros were used for this purpose, but
+those macros hide return statements, and should not be used in new code.
+(This coding style rule isn't generally agreed, so use of ``NS_ENSURE_*``
+can be valid.)
+
+
+Return from errors immediately
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+In most cases, your knee-jerk reaction should be to return from the
+current function, when an error condition occurs. Don't do this:
+
+.. code-block:: cpp
+
+   rv = foo->Call1();
+   if (NS_SUCCEEDED(rv)) {
+     rv = foo->Call2();
+     if (NS_SUCCEEDED(rv)) {
+       rv = foo->Call3();
+     }
+   }
+   return rv;
+
+Instead, do this:
+
+.. code-block:: cpp
+
+   rv = foo->Call1();
+   if (NS_FAILED(rv)) {
+     return rv;
+   }
+
+   rv = foo->Call2();
+   if (NS_FAILED(rv)) {
+     return rv;
+   }
+
+   rv = foo->Call3();
+   if (NS_FAILED(rv)) {
+     return rv;
+   }
+
+Why? Error handling should not obfuscate the logic of the code. The
+author's intent, in the first example, was to make 3 calls in
+succession. Wrapping the calls in nested if() statements, instead
+obscured the most likely behavior of the code.
+
+Consider a more complicated example to hide a bug:
+
+.. code-block:: cpp
+
+   bool val;
+   rv = foo->GetBooleanValue(&val);
+   if (NS_SUCCEEDED(rv) && val) {
+     foo->Call1();
+   } else {
+     foo->Call2();
+   }
+
+The intent of the author, may have been, that ``foo->Call2()`` would only
+happen when val had a false value. In fact, ``foo->Call2()`` will also be
+called, when ``foo->GetBooleanValue(&val)`` fails. This may, or may not,
+have been the author's intent. It is not clear from this code. Here is
+an updated version:
+
+.. code-block:: cpp
+
+   bool val;
+   rv = foo->GetBooleanValue(&val);
+   if (NS_FAILED(rv)) {
+     return rv;
+   }
+   if (val) {
+     foo->Call1();
+   } else {
+     foo->Call2();
+   }
+
+In this example, the author's intent is clear, and an error condition
+avoids both calls to ``foo->Call1()`` and ``foo->Call2();``
+
+*Possible exceptions:* Sometimes it is not fatal if a call fails. For
+instance, if you are notifying a series of observers that an event has
+fired, it might be trivial that one of these notifications failed:
+
+.. code-block:: cpp
+
+   for (size_t i = 0; i < length; ++i) {
+     // we don't care if any individual observer fails
+     observers[i]->Observe(foo, bar, baz);
+   }
+
+Another possibility, is you are not sure if a component exists or is
+installed, and you wish to continue normally, if the component is not
+found.
+
+.. code-block:: cpp
+
+   nsCOMPtr<nsIMyService> service = do_CreateInstance(NS_MYSERVICE_CID, &rv);
+   // if the service is installed, then we'll use it.
+   if (NS_SUCCEEDED(rv)) {
+     // non-fatal if this fails too, ignore this error.
+     service->DoSomething();
+
+     // this is important, handle this error!
+     rv = service->DoSomethingImportant();
+     if (NS_FAILED(rv)) {
+       return rv;
+     }
+   }
+
+   // continue normally whether or not the service exists.
+
+
+Strings
+-------
+
+.. note::
+
+   This section overlaps with the more verbose advice given in
+   :doc:`String guide </xpcom/stringguide>`.
+   These should eventually be merged. For now, please refer to that guide for
+   more advice.
+
+-  String arguments to functions should be declared as ``[const] nsA[C]String&``.
+-  Prefer using string literals. In particular, use empty string literals,
+   i.e. ``u""_ns`` or ``""_ns``, instead of ``Empty[C]String()`` or
+   ``const nsAuto[C]String empty;``. Use ``Empty[C]String()`` only if you
+   specifically need a ``const ns[C]String&``, e.g. with the ternary operator
+   or when you need to return/bind to a reference or take the address of the
+   empty string.
+-  For 16-bit literal strings, use ``u"..."_ns`` or, if necessary
+   ``NS_LITERAL_STRING_FROM_CSTRING(...)`` instead of ``nsAutoString()``
+   or other ways that would do a run-time conversion.
+   See :ref:`Avoid runtime conversion of string literals <Avoid runtime conversion of string literals>` below.
+-  To compare a string with a literal, use ``.EqualsLiteral("...")``.
+-  Use ``str.IsEmpty()`` instead of ``str.Length() == 0``.
+-  Use ``str.Truncate()`` instead of ``str.SetLength(0)``,
+   ``str.Assign(""_ns)`` or ``str.AssignLiteral("")``.
+-  Don't use functions from ``ctype.h`` (``isdigit()``, ``isalpha()``,
+   etc.) or from ``strings.h`` (``strcasecmp()``, ``strncasecmp()``).
+   These are locale-sensitive, which makes them inappropriate for
+   processing protocol text. At the same time, they are too limited to
+   work properly for processing natural-language text. Use the
+   alternatives in ``mozilla/TextUtils.h`` and in ``nsUnicharUtils.h``
+   in place of ``ctype.h``. In place of ``strings.h``, prefer the
+   ``nsStringComparator`` facilities for comparing strings or if you
+   have to work with zero-terminated strings, use ``nsCRT.h`` for
+   ASCII-case-insensitive comparison.
+
+
+Use the ``Auto`` form of strings for local values
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+When declaring a local, short-lived ``nsString`` class, always use
+``nsAutoString`` or ``nsAutoCString``. These pre-allocate a 64-byte
+buffer on the stack, and avoid fragmenting the heap. Don't do this:
+
+.. code-block:: cpp
+
+   nsresult
+   foo()
+   {
+     nsCString bar;
+     ..
+   }
+
+instead:
+
+.. code-block:: cpp
+
+   nsresult
+   foo()
+   {
+     nsAutoCString bar;
+     ..
+   }
+
+
+Be wary of leaking values from non-XPCOM functions that return char\* or PRUnichar\*
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+It is an easy trap to return an allocated string, from an internal
+helper function, and then using that function inline in your code,
+without freeing the value. Consider this code:
+
+.. code-block:: cpp
+
+   static char*
+   GetStringValue()
+   {
+     ..
+     return resultString.ToNewCString();
+   }
+
+     ..
+     WarnUser(GetStringValue());
+
+In the above example, ``WarnUser`` will get the string allocated from
+``resultString.ToNewCString()`` and throw away the pointer. The
+resulting value is never freed. Instead, either use the string classes,
+to make sure your string is automatically freed when it goes out of
+scope, or make sure that your string is freed.
+
+Automatic cleanup:
+
+.. code-block:: cpp
+
+   static void
+   GetStringValue(nsAWritableCString& aResult)
+   {
+     ..
+     aResult.Assign("resulting string");
+   }
+
+     ..
+     nsAutoCString warning;
+     GetStringValue(warning);
+     WarnUser(warning.get());
+
+Free the string manually:
+
+.. code-block:: cpp
+
+   static char*
+   GetStringValue()
+   {
+     ..
+     return resultString.ToNewCString();
+   }
+
+     ..
+     char* warning = GetStringValue();
+     WarnUser(warning);
+     nsMemory::Free(warning);
+
+.. _Avoid runtime conversion of string literals:
+
+Avoid runtime conversion of string literals
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+It is very common to need to assign the value of a literal string, such
+as ``"Some String"``, into a unicode buffer. Instead of using ``nsString``'s
+``AssignLiteral`` and ``AppendLiteral``, use a user-defined literal like `u"foo"_ns`
+instead. On most platforms, this will force the compiler to compile in a
+raw unicode string, and assign it directly. In cases where the literal is defined
+via a macro that is used in both 8-bit and 16-bit ways, you can use
+`NS_LITERAL_STRING_FROM_CSTRING` to do the conversion at compile time.
+
+Incorrect:
+
+.. code-block:: cpp
+
+   nsAutoString warning;
+   warning.AssignLiteral("danger will robinson!");
+   ...
+   foo->SetStringValue(warning);
+   ...
+   bar->SetUnicodeValue(warning.get());
+
+Correct:
+
+.. code-block:: cpp
+
+   constexpr auto warning = u"danger will robinson!"_ns;
+   ...
+   // if you'll be using the 'warning' string, you can still use it as before:
+   foo->SetStringValue(warning);
+   ...
+   bar->SetUnicodeValue(warning.get());
+
+   // alternatively, use the wide string directly:
+   foo->SetStringValue(u"danger will robinson!"_ns);
+   ...
+
+   // if a macro is the source of a 8-bit literal and you cannot change it, use
+   // NS_LITERAL_STRING_FROM_CSTRING, but only if necessary.
+   #define MY_MACRO_LITERAL "danger will robinson!"
+   foo->SetStringValue(NS_LITERAL_STRING_FROM_CSTRING(MY_MACRO_LITERAL));
+
+   // If you need to pass to a raw const char16_t *, there's no benefit to
+   // go through our string classes at all, just do...
+   bar->SetUnicodeValue(u"danger will robinson!");
+
+   // .. or, again, if a macro is the source of a 8-bit literal
+   bar->SetUnicodeValue(u"" MY_MACRO_LITERAL);
+
+
+Usage of PR_(MAX|MIN|ABS|ROUNDUP) macro calls
+---------------------------------------------
+
+Use the standard-library functions (``std::max``), instead of
+``PR_(MAX|MIN|ABS|ROUNDUP)``.
+
+Use ``mozilla::Abs`` instead of ``PR_ABS``. All ``PR_ABS`` calls in C++ code have
+been replaced with ``mozilla::Abs`` calls, in `bug
+847480 <https://bugzilla.mozilla.org/show_bug.cgi?id=847480>`__. All new
+code in ``Firefox/core/toolkit`` needs to ``#include "nsAlgorithm.h"`` and
+use the ``NS_foo`` variants instead of ``PR_foo``, or
+``#include "mozilla/MathAlgorithms.h"`` for ``mozilla::Abs``.
+
+Use of SpiderMonkey rooting typedefs
+------------------------------------
+The rooting typedefs in ``js/public/TypeDecls.h``, such as ``HandleObject`` and
+``RootedObject``, are deprecated both in and outside of SpiderMonkey. They will
+eventually be removed and should not be used in new code.
diff --git a/docs/code-quality/coding-style/coding_style_general.rst b/docs/code-quality/coding-style/coding_style_general.rst
new file mode 100644
index 0000000000..950cd6ccd3
--- /dev/null
+++ b/docs/code-quality/coding-style/coding_style_general.rst
@@ -0,0 +1,18 @@
+
+Mode line
+~~~~~~~~~
+
+Files should have Emacs and vim mode line comments as the first two
+lines of the file, which should set ``indent-tabs-mode`` to ``nil``. For new
+files, use the following, specifying two-space indentation:
+
+.. code-block:: cpp
+
+   /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+   /* vim: set ts=2 et sw=2 tw=80: */
+   /* This Source Code Form is subject to the terms of the Mozilla Public
+    * License, v. 2.0. If a copy of the MPL was not distributed with this
+    * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+Be sure to use the correct ``Mode`` in the first line, don't use ``C++`` in
+JavaScript files.
diff --git a/docs/code-quality/coding-style/coding_style_java.rst b/docs/code-quality/coding-style/coding_style_java.rst
new file mode 100644
index 0000000000..f2206d8e2d
--- /dev/null
+++ b/docs/code-quality/coding-style/coding_style_java.rst
@@ -0,0 +1,68 @@
+=================
+Java Coding style
+=================
+
+-  We use the `Java Coding
+   Style <https://www.oracle.com/technetwork/java/codeconvtoc-136057.html>`__.
+   Quick summary:
+
+   -  FirstLetterUpperCase for class names.
+   -  camelCase for method and variable names.
+   -  One declaration per line:
+
+      .. code-block:: java
+
+         int x, y; // this is BAD!
+         int a;    // split it over
+         int b;    // two lines
+
+-  Braces should be placed like so (generally, opening braces on same
+   line, closing braces on a new line):
+
+   .. code-block:: java
+
+      public void func(int arg) {
+          if (arg != 0) {
+              while (arg > 0) {
+                  arg--;
+              }
+          } else {
+              arg++;
+          }
+      }
+
+-  Places we differ from the Java coding style:
+
+   -  Start class variable names with 'm' prefix (e.g.
+      mSomeClassVariable) and static variables with 's' prefix (e.g.
+      sSomeStaticVariable)
+   -  ``import`` statements:
+
+      -  Do not use wildcard imports like \`import java.util.*;\`
+      -  Organize imports by blocks separated by empty line:
+         org.mozilla.*, android.*, com.*, net.*, org.*, then java.\*
+         This is basically what Android Studio does by default, except
+         that we place org.mozilla.\* at the front - please adjust
+         Settings -> Editor -> Code Style -> Java -> Imports
+         accordingly.
+      -  Within each import block, alphabetize import names with
+         uppercase before lowercase. For example, ``com.example.Foo`` is
+         before ``com.example.bar``
+
+   -  4-space indents.
+   -  Spaces, not tabs.
+   -  Don't restrict yourself to 80-character lines. Google's Android
+      style guide suggests 100-character lines, which is also the
+      default setting in Android Studio. Java code tends to be long
+      horizontally, so use appropriate judgement when wrapping. Avoid
+      deep indents on wrapping. Note that aligning the wrapped part of a
+      line, with some previous part of the line (rather than just using
+      a fixed indent), may require shifting the code every time the line
+      changes, resulting in spurious whitespace changes.
+
+-  For additional specifics on Firefox for Android, see the `Coding
+   Style guide for Firefox on
+   Android <https://wiki.mozilla.org/Mobile/Fennec/Android#Coding_Style>`__.
+-  The `Android Coding
+   Style <https://source.android.com/source/code-style.html>`__ has some
+   useful guidelines too.
diff --git a/docs/code-quality/coding-style/coding_style_js.rst b/docs/code-quality/coding-style/coding_style_js.rst
new file mode 100644
index 0000000000..09d7a6fc8a
--- /dev/null
+++ b/docs/code-quality/coding-style/coding_style_js.rst
@@ -0,0 +1,147 @@
+=======================
+JavaScript Coding style
+=======================
+
+Coding style
+~~~~~~~~~~~~
+
+`prettier <https://prettier.io/>`_ is the tool used to reformat the JavaScript code.
+
+
+Methods and functions
+~~~~~~~~~~~~~~~~~~~~~
+
+In JavaScript, functions should use camelCase, but should not capitalize
+the first letter. Methods should not use the named function expression
+syntax, because our tools understand method names:
+
+.. code-block:: cpp
+
+   doSomething: function (aFoo, aBar) {
+     ...
+   }
+
+In-line functions should have spaces around braces, except before commas
+or semicolons:
+
+.. code-block:: cpp
+
+   function valueObject(aValue) { return { value: aValue }; }
+
+
+JavaScript objects
+~~~~~~~~~~~~~~~~~~
+
+.. code-block:: cpp
+
+   var foo = { prop1: "value1" };
+
+   var bar = {
+     prop1: "value1",
+     prop2: "value2"
+   };
+
+Constructors for objects should be capitalized and use Pascal Case:
+
+.. code-block:: cpp
+
+   function ObjectConstructor() {
+     this.foo = "bar";
+   }
+
+
+Operators
+~~~~~~~~~
+
+In JavaScript, overlong expressions not joined by ``&&`` and
+``||`` should break so the operator starts on the second line and
+starting in the same column as the beginning of the expression in the
+first line. This applies to ``?:``, binary arithmetic operators
+including ``+``, and member-of operators. Rationale: an operator at the
+front of the continuation line makes for faster visual scanning, as
+there is no need to read to the end of line. Also there exists a
+context-sensitive keyword hazard in JavaScript; see {{bug(442099, "bug",
+19)}}, which can be avoided by putting . at the start of a continuation
+line, in long member expression.
+
+In JavaScript, ``==`` is preferred to ``===``.
+
+Unary keyword operators, such as ``typeof``, should have their operand
+parenthesized; e.g. ``typeof("foo") == "string"``.
+
+Literals
+~~~~~~~~
+
+Double-quoted strings (e.g. ``"foo"``) are preferred to single-quoted
+strings (e.g. ``'foo'``), in JavaScript, except to avoid escaping
+embedded double quotes, or when assigning inline event handlers.
+
+
+Prefixes
+~~~~~~~~
+
+-  k=constant (e.g. ``kNC_child``). Not all code uses this style; some
+   uses ``ALL_CAPS`` for constants.
+-  g=global (e.g. ``gPrefService``)
+-  a=argument (e.g. ``aCount``)
+
+-  JavaScript Specific Prefixes
+
+   -  \_=member (variable or function) (e.g. ``_length`` or
+      ``_setType(aType)``)
+   -  k=enumeration value (e.g. ``const kDisplayModeNormal = 0``)
+   -  on=event handler (e.g. ``function onLoad()``)
+   -  Convenience constants for interface names should be prefixed with
+      ``nsI``:
+
+      .. code-block:: javascript
+
+         const nsISupports = Components.interfaces.nsISupports;
+         const nsIWBN = Components.interfaces.nsIWebBrowserNavigation;
+
+
+
+Other advices
+~~~~~~~~~~~~~
+
+-  Do not compare ``x == true`` or ``x == false``. Use ``(x)`` or
+   ``(!x)`` instead. ``x == true``, is certainly different from if
+   ``(x)``! Compare objects to ``null``, numbers to ``0`` or strings to
+   ``""``, if there is chance for confusion.
+-  Make sure that your code doesn't generate any strict JavaScript
+   warnings, such as:
+
+   -  Duplicate variable declaration.
+   -  Mixing ``return;`` with ``return value;``
+   -  Undeclared variables or members. If you are unsure if an array
+      value exists, compare the index to the array's length. If you are
+      unsure if an object member exists, use ``"name"`` in ``aObject``,
+      or if you are expecting a particular type you may use
+      ``typeof(aObject.name) == "function"`` (or whichever type you are
+      expecting).
+
+-  Use ``['value1, value2']`` to create a JavaScript array in preference
+   to using
+   ``new {{JSxRef("Array", "Array", "Syntax", 1)}}(value1, value2)``
+   which can be confusing, as ``new Array(length)`` will actually create
+   a physically empty array with the given logical length, while
+   ``[value]`` will always create a 1-element array. You cannot actually
+   guarantee to be able to preallocate memory for an array.
+-  Use ``{ member: value, ... }`` to create a JavaScript object; a
+   useful advantage over ``new {{JSxRef("Object", "Object", "", 1)}}()``
+   is the ability to create initial properties and use extended
+   JavaScript syntax, to define getters and setters.
+-  If having defined a constructor you need to assign default
+   properties, it is preferred to assign an object literal to the
+   prototype property.
+-  Use regular expressions, but use wisely. For instance, to check that
+   ``aString`` is not completely whitespace use
+   ``/\S/.{{JSxRef("RegExp.test", "test(aString)", "", 1)}}``. Only use
+   {{JSxRef("String.search", "aString.search()")}} if you need to know
+   the position of the result, or {{JSxRef("String.match",
+   "aString.match()")}} if you need to collect matching substrings
+   (delimited by parentheses in the regular expression). Regular
+   expressions are less useful if the match is unknown in advance, or to
+   extract substrings in known positions in the string. For instance,
+   {{JSxRef("String.slice", "aString.slice(-1)")}} returns the last
+   letter in ``aString``, or the empty string if ``aString`` is empty.
diff --git a/docs/code-quality/coding-style/coding_style_python.rst b/docs/code-quality/coding-style/coding_style_python.rst
new file mode 100644
index 0000000000..3a818fcfd4
--- /dev/null
+++ b/docs/code-quality/coding-style/coding_style_python.rst
@@ -0,0 +1,71 @@
+===================
+Python Coding style
+===================
+
+Coding style
+~~~~~~~~~~~~
+
+ :ref:`black` is the tool used to reformat the Python code.
+
+Linting
+~~~~~~~
+
+The Python linting is done by :ref:`Flake8` and :ref:`pylint`
+They are executed by mozlint both at review phase and in the CI.
+
+Indentation
+~~~~~~~~~~~
+
+Four spaces in Python code.
+
+
+Makefile/moz.build practices
+----------------------------
+
+-  Changes to makefile and moz.build variables do not require
+   build-config peer review. Any other build system changes, such as
+   adding new scripts or rules, require review from the build-config
+   team.
+-  Suffix long ``if``/``endif`` conditionals with #{ & #}, so editors
+   can display matched tokens enclosing a block of statements.
+
+   ::
+
+      ifdef CHECK_TYPE #{
+        ifneq ($(flavor var_type),recursive) #{
+          $(warning var should be expandable but detected var_type=$(flavor var_type))
+        endif #}
+      endif #}
+
+-  moz.build are python and follow normal Python style.
+-  List assignments should be written with one element per line. Align
+   closing square brace with start of variable assignment. If ordering
+   is not important, variables should be in alphabetical order.
+
+   .. code-block:: python
+
+      var += [
+          'foo',
+          'bar'
+      ]
+
+-  Use ``CONFIG['CPU_ARCH'] {=arm}`` to test for generic classes of
+   architecture rather than ``CONFIG['OS_TEST'] {=armv7}`` (re: bug 886689).
+
+
+Other advices
+~~~~~~~~~~~~~
+
+-  Install the
+   `mozext <https://hg.mozilla.org/hgcustom/version-control-tools/file/default/hgext/mozext>`__
+   Mercurial extension, and address every issue reported on commit
+   or the output of ``hg critic``.
+-  Follow `PEP 8 <https://www.python.org/dev/peps/pep-0008/>`__. Please run :ref:`black` for this.
+-  Do not place statements on the same line as ``if/elif/else``
+   conditionals to form a one-liner.
+-  Global vars, please avoid them at all cost.
+-  Exclude outer parenthesis from conditionals.Use
+   ``if x > 5:,``\ rather than ``if (x > 5):``
+-  Use string formatters, rather than var + str(val).
+   ``var = 'Type %s value is %d'% ('int', 5).``
+-  Testing/Unit tests, please write them and make sure that they are executed in the CI.
diff --git a/docs/code-quality/coding-style/css_guidelines.rst b/docs/code-quality/coding-style/css_guidelines.rst
new file mode 100644
index 0000000000..e962f24d69
--- /dev/null
+++ b/docs/code-quality/coding-style/css_guidelines.rst
@@ -0,0 +1,572 @@
+CSS Guidelines
+==============
+
+This document contains guidelines defining how CSS inside the Firefox
+codebase should be written, it is notably relevant for Firefox front-end
+engineers.
+
+Basics
+------
+
+Here are some basic tips that can optimize reviews if you are changing
+CSS:
+
+-  Avoid ``!important`` but if you have to use it, make sure it's
+   obvious why you're using it (ideally with a comment). The
+   `Overriding CSS`_ section contains more information about this.
+-  Avoid magic numbers; prefer automatic sizing or alignment methods.
+   Some examples to avoid:
+
+   -  absolutely positioned elements
+   -  hardcoded values such as: ``vertical-align: -2px;`` . The reason
+      you should avoid such "hardcoded" values is that, they don't
+      necessarily work for all font-size configurations.
+
+-  Avoid setting styles in JavaScript. It's generally better to set a
+   class and then specify the styles in CSS.
+-  ``classList`` is generally better than ``className``. There's less
+   chance of overwriting an existing class.
+-  Only use generic selectors such as ``:last-child``, when it is what
+   you mean semantically. If not, using a semantic class name is more
+   descriptive and usually better.
+
+Boilerplate
+~~~~~~~~~~~
+
+Make sure each file starts with the standard copyright header (see
+`License Boilerplate <https://www.mozilla.org/MPL/headers/>`__).
+
+Before adding more CSS
+~~~~~~~~~~~~~~~~~~~~~~
+
+It is good practice to check if the CSS that is being written is needed,
+it can be the case that a common component has been already written
+could be reused with or without changes. Most of the time, the common
+component already follows the a11y/theme standards defined in this
+guide. So, when possible, always prefer editing common components to
+writing your own.
+
+Also, it is good practice to introduce a common class when the new
+element you are styling reuses some styles from another element, this
+allows the maintenance cost and the amount of code duplication to be
+reduced.
+
+Formatting
+----------
+
+Spacing & Indentation
+~~~~~~~~~~~~~~~~~~~~~
+
+-  2 spaces indentation is preferred
+-  Add a space after each comma, **except** within color functions:
+
+.. code:: css
+
+   linear-gradient(to bottom, black 1px, rgba(255,255,255,0.2) 1px)
+
+-  Always add a space before ``!important``.
+
+Omit units on 0 values
+~~~~~~~~~~~~~~~~~~~~~~
+
+Do this:
+
+.. code:: css
+
+     margin: 0;
+
+Not this:
+
+.. code:: css
+
+     margin: 0px;
+
+Use expanded syntax
+~~~~~~~~~~~~~~~~~~~
+
+It is often harder to understand what the shorthand is doing and the
+shorthand can also hide some unwanted default values. It is good to
+privilege expanded syntax to make your intentions explicit.
+
+Do this:
+
+.. code:: css
+
+     border-color: red;
+
+Not this:
+
+.. code:: css
+
+     border: red;
+
+Put multiple selectors on different lines
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Do this:
+
+.. code:: css
+
+   h1,
+   h2,
+   h3 {
+     font-family: sans-serif;
+     text-align: center;
+   }
+
+Not this:
+
+.. code:: css
+
+   h1, h2, h3 {
+     font-family: sans-serif;
+     text-align: center;
+   }
+
+Naming standards for class names
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+-  ``lower-case-with-dashes`` is the most common.
+-  But ``camelCase`` is also used sometimes. Try to follow the style of
+   existing or related code.
+
+Other tips
+~~~~~~~~~~
+
+-  Assume ``="true"`` in attribute selectors.
+
+   -  Example: Use ``option[checked]``, not ``option[checked="true"]``.
+
+-  Avoid ID selectors unless it is really the wanted goal, since IDs
+   have higher specificity and therefore are harder to override.
+-  Using descendant selectors is good practice for performance when
+   possible:
+
+   -  For example:
+      ``.autocomplete-item[selected] > .autocomplete-item-title`` would
+      be more efficient than
+      ``.autocomplete-item[selected] .autocomplete-item-title``
+
+Overriding CSS
+--------------
+
+Before overriding any CSS rules, check whether overriding is really
+needed. Sometimes, when copy-pasting older code, it happens that the
+code in question contains unnecessary overrides. This could be because
+the CSS that it was overriding got removed in the meantime. In this
+case, dropping the override should work.
+
+It is also good practice to look at whether the rule you are overriding
+is still needed: maybe the UX spec for the component has changed and
+that rule can actually be updated or removed. When this is the case,
+don't be afraid to remove or update that rule.
+
+Once the two things above have been checked, check if the other rule you
+are overriding contains ``!important``, if that is case, try putting it
+in question, because it might have become obsolete.
+
+Afterwards, check the specificity of the other selector; if it is
+causing your rule to be overridden, you can try reducing its
+specificity, either by simplifying the selector or by changing where the
+rule is placed in the stylesheet. If this isn't possible, you can also
+try introducing a ``:not()`` to prevent the other rule from applying,
+this is especially relevant for different element states (``:hover``,
+``:active``, ``[checked]`` or ``[disabled]``). However, never try to
+increase the selector of the rule you are adding as it can easily become
+hard to understand.
+
+Finally, once you have checked all the things above, you can permit
+yourself to use ``!important`` along with a comment why it is needed.
+
+Using CSS variables
+-------------------
+
+Adding new variables
+~~~~~~~~~~~~~~~~~~~~
+
+Before adding new CSS variables, please consider the following
+questions:
+
+#. **Is the variable value changed at runtime?**
+   *(Either from JavaScript or overridden by another CSS file)*
+   **If the answer is no**, consider using a preprocessor variable or
+   inlining the value.
+
+#. **Is the variable value used multiple times?**
+   **If the answer is no and the value isn't changed at runtime**, then
+   you likely don't need a CSS variable.
+
+#. **Is there an alternative to using the variable like inheriting or
+   using the ``currentcolor`` keyword?**
+   Using inheriting or using ``currentcolor`` will prevent repetition of
+   the value and it is usually good practice to do so.
+
+In general, it's good to first think of how some CSS could be written
+cleanly without the CSS variable(s) and then think of how the CSS
+variable could improve that CSS.
+
+Using variables
+~~~~~~~~~~~~~~~
+
+Use the variable according to its naming
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Do this:
+
+.. code:: css
+
+   xul|tab:hover {
+     background-color: var(--in-content-box-background-hover);
+   }
+
+Not this:
+
+.. code:: css
+
+   #certificateErrorDebugInformation {
+     background-color: var(--in-content-box-background-hover);
+   }
+
+Localization
+------------
+
+Text Direction
+~~~~~~~~~~~~~~
+
+-  For margins, padding and borders, use
+   ``inline-start``/``inline-end`` rather than ``left``/``right``.
+   *Example:* Use ``margin-inline-start: 3px;`` instead of
+   ``margin-left: 3px``.
+-  For RTL-aware positioning (left/right), use
+   ``inset-inline-start``/``inset-inline-end``.
+-  For RTL-aware float layouts, ``float: inline-start|inline-end`` can
+   be used instead of ``float: left|right``.
+-  The RTL-aware equivalents of
+   ``border-{top/bottom}-{left/right}-radius`` are
+   ``border-{start/end}-{start/end}-radius``
+-  When there is no special RTL-aware property available, use the pseudo
+   ``:-moz-locale-dir(ltr|rtl)`` (for XUL files) or ``:dir(ltr|rtl)``
+   (for HTML files).
+-  Remember that while a tab content's scrollbar still shows on the
+   right in RTL, an overflow scrollbar will show on the left.
+-  Write ``padding: 0 3px 4px;`` instead of
+   ``padding: 0 3px 4px 3px;``. This makes it more obvious that the
+   padding is symmetrical (so RTL won't be an issue).
+
+.. note::
+
+   See `CSS Logical Properties and
+   Values <https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Logical_Properties>`__
+   for more information.
+
+Testing
+~~~~~~~
+
+To test for RTL layouts, you can go to ``about:config`` and set
+``intl.uidirection`` to ``-1``.
+
+Writing cross-platform CSS
+--------------------------
+
+Firefox supports many different platforms and each of those platforms
+can contain many different configurations:
+
+-  Windows 7, 8 and 10
+
+   -  Default theme
+   -  Aero basic (Windows 7, 8)
+   -  Windows classic (Windows 7)
+   -  High contrast (All versions)
+
+-  Linux
+-  macOS
+
+File structure
+~~~~~~~~~~~~~~
+
+-  The ``browser/`` directory contains styles specific to Firefox
+-  The ``toolkit/`` directory contains styles that are shared across all
+   toolkit applications (Thunderbird and SeaMonkey)
+
+Under each of those two directories, there is a ``themes`` directory
+containing 4 sub-directories:
+
+-  ``shared``
+-  ``linux``
+-  ``osx``
+-  ``windows``
+
+The ``shared`` directories contain styles shared across all 3 platforms,
+while the other 3 directories contain styles respective to their
+platform.
+
+For new CSS, when possible try to privilege using the ``shared``
+directory, instead of writing the same CSS for the 3 platform specific
+directories, especially for large blocks of CSS.
+
+Content CSS vs. Theme CSS
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The following directories also contain CSS:
+
+-  ``browser/base/content/``
+-  ``toolkit/content/``
+
+These directories contain content CSS, that applies on all platforms,
+which is styling deemed to be essential for the browser to behave
+correctly. To determine whether some CSS is theme-side or content-side,
+it is useful to know that certain CSS properties are going to lean one
+way or the other: color - 99% of the time it will be theme CSS, overflow
+- 99% content.
+
++-----------------+--------------+----------------+----------------+
+| 99% theme       | 70% theme    | 70% content    | 99% content    |
++=================+==============+================+================+
+| font-\*, color, | line-height, | cursor, width, | overflow,      |
+| \*-color,       | padding,     | max-width,     | direction,     |
+| border-\*,      | margin       | top,           | display,       |
+| -moz-appearance |              | bottom [2]_,   | \*-align,      |
+| [1]_            |              | etc            | align-\*,      |
+|                 |              |                | \*-box-\*,     |
+|                 |              |                | flex-\*, order |
++-----------------+--------------+----------------+----------------+
+
+If some CSS is layout or functionality related, then it is likely
+content CSS. If it is esthetics related, then it is likely theme CSS.
+
+When importing your stylesheets, it's best to import the content CSS
+before the theme CSS, that way the theme values get to override the
+content values (which is probably what you want), and you're going to
+want them both after the global values, so your imports will look like
+this:
+
+.. code:: html
+
+   <?xml-stylesheet href="chrome://global/skin/global.css" type="text/css"?>
+   <?xml-stylesheet href="chrome://browser/content/path/module.css" type="text/css"?>
+   <?xml-stylesheet href="chrome://browser/skin/path/module.css" type="text/css"?>
+
+.. [1] -moz-appearance is tricky. Generally, when specifying
+   -moz-appearance: foo; you're giving hints as to how something should
+   act, however -moz-appearance: none; is probably saying 'ignore
+   browser preconceptions - I want a blank sheet', so that's more
+   visual. However -moz-appearance values aren't implemented and don't
+   behave consistently across platforms, so idealism aside
+   -moz-appearance should always be in theme CSS.
+
+.. [2] However there is probably a better way than using absolute
+   positioning.
+
+Colors
+~~~~~~
+
+For common areas of the Firefox interface (panels, toolbar buttons,
+etc.), mozilla-central often comes with some useful CSS variables that
+are adjusted with the correct values for different platform
+configurations, so using those CSS variables can definitively save some
+testing time, as you can assume they already work correctly.
+
+Using the ``currentcolor`` keyword or inheriting is also good practice,
+because sometimes the needed value is already in the color or on the
+parent element. This is especially useful in conjunction with icons
+using ``-moz-context-properties: fill;`` where the icon can adjust to
+the right platform color automatically from the text color. It is also
+possible to use ``currentcolor`` with other properties like
+``opacity`` or ``fill-opacity`` to have different
+opacities of the platform color.
+
+High contrast mode
+~~~~~~~~~~~~~~~~~~
+
+Content area
+^^^^^^^^^^^^
+
+On Windows high contrast mode, in the content area, Gecko does some
+automatic color adjustments regarding page colors. Part of those
+adjustments include making all ``box-shadow`` invisible, so this is
+something to be aware of if you create a focus ring or a border using
+the ``box-shadow`` property: consider using a ``border`` or an
+``outline`` if you want the border/focus ring to stay visible in
+high-contrast mode. An example of such bug is `bug
+1516767 <https://bugzilla.mozilla.org/show_bug.cgi?id=1516767>`__.
+
+Another adjustment to be aware of is that Gecko removes all the
+``background-image`` when high contrast mode is enabled. Consider using
+an actual ``<img>`` tag (for HTML documents) or ``list-style-image``
+(for XUL documents) if rendering the image is important.
+
+If you are not using Windows, one way to test against those adjustments
+on other platforms is:
+
+-  Going to about:preferences
+-  Clicking on the "Colors..." button in the "Fonts & Colors"
+   sub-section of the "Language and Appearance" section
+-  Under "Override the colors specified by the page with your selections
+   above", select the "Always" option
+
+Chrome area
+^^^^^^^^^^^
+
+The automatic adjustments previously mentioned only apply to pages
+rendered in the content area. The chrome area of Firefox uses colors as
+authored, which is why using pre-defined variables, ``currentcolor`` or
+inheritance is useful to integrate with the system theme with little
+hassle.
+
+If not, as a last resort, using `system
+colors <https://developer.mozilla.org/en-US/docs/Web/CSS/color_value#system_colors>`__
+also works for non-default Windows themes or Linux. In general, the
+following colors are used:
+
+-  ``-moz-Field``: textbox or field background colors, also used as the
+   background color of listboxes or trees.
+-  ``-moz-FieldText``: textbox or field text colors, also used as the
+   text color of listboxes or trees.
+-  ``-moz-Dialog``: window or dialog background color.
+-  ``-moz-DialogText``: window or dialog text color.
+-  ``GrayText``: used on disabled items as text color. Do not use it on
+   text that is not disabled to desemphsize text, because it does not
+   guarantee a sufficient contrast ratio for non-disabled text.
+-  ``ThreeDShadow``: Used as border on elements.
+-  ``ThreeDLightShadow``: Used as light border on elements.
+
+Using the background/text pairs is especially important to ensure the
+contrast is respected in all situations. Never mix custom text colors
+with a system background color and vice-versa.
+
+Note that using system colors is only useful for the chrome area, since
+content area colors are overridden by Gecko anyway.
+
+Writing media queries
+~~~~~~~~~~~~~~~~~~~~~
+
+Boolean media queries
+^^^^^^^^^^^^^^^^^^^^^
+
+Do this:
+
+.. code:: css
+
+   @media (-moz-mac-yosemite-theme: 0) {
+
+Not this:
+
+.. code:: css
+
+   @media not all and (-moz-mac-yosemite-theme) {
+
+Privilege CSS for most common configuration
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+It is better to put the most common configuration (latest version of an
+OS, or default theme for example) outside of the media query. In the
+following example, ``-moz-mac-yosemite-theme`` targets macOS 10.10 and
+higher, so it should be privileged over the styling for macOS 10.9.
+
+Do this:
+
+.. code:: css
+
+   @media (-moz-mac-yosemite-theme: 0) {
+     #placesList {
+       box-shadow: inset -2px 0 0 hsla(0,0%,100%,.2);
+     }
+   }
+
+Not this:
+
+.. code:: css
+
+   #placesList {
+     box-shadow: inset -2px 0 0 hsla(0,0%,100%,.2);
+   }
+
+   @media (-moz-mac-yosemite-theme) {
+     #placesList {
+       box-shadow: none;
+     }
+   }
+
+Theme support
+-------------
+
+Firefox comes built-in with 3 themes: default, light and dark. The
+built-in light/dark themes are a bit special as they load the
+``compacttheme.css`` stylesheet. In addition to this, Firefox supports a
+variety of WebExtension themes that can be installed from AMO. For
+testing purposes, `here is an example of a WebExtension
+theme. <https://addons.mozilla.org/en-US/firefox/addon/arc-dark-theme-we/>`__
+
+Writing theme-friendly CSS
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+-  Some CSS variables that are pre-adjusted for different platforms are
+   also pre-adjusted for themes, so it's again a good idea to use them
+   for theme support.
+-  The text color of elements often contains valuable information from
+   the theme colors, so ``currentcolor``/inheritance is again a good
+   idea for theme support.
+-  Never write CSS specially for the built-in light/dark theme in
+   ``compacttheme.css`` unless that CSS isn't supposed to affect
+   WebExtension themes.
+-  These selectors can be used to target dark areas:
+
+   -  ``:-moz-lwtheme-brighttext``: dark window frame.
+   -  ``:root[lwt-toolbar-field-brighttext]``: dark address bar and
+      searchbar.
+   -  ``:root[lwt-popup-brighttext]``: dark arrow panels and
+      autocomplete panels.
+   -  ``:root[lwt-sidebar-brighttext]``: dark sidebars.
+
+-  If you'd like a different shade of a themed area and no CSS variable
+   is adequate, using colors with alpha transparency is usually a good
+   idea, as it will preserve the original theme author's color hue.
+
+Variables
+~~~~~~~~~
+
+For clarity, CSS variables that are only used when a theme is enabled
+have the ``--lwt-`` prefix.
+
+Layout & performance
+--------------------
+
+Layout
+~~~~~~
+
+Mixing XUL flexbox and HTML flexbox can lead to undefined behavior.
+
+CSS selectors
+~~~~~~~~~~~~~
+
+When targeting the root element of a page, using ``:root`` is the most
+performant way of doing so.
+
+Reflows and style flushes
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+See :ref:`Performance best practices for Firefox front-end engineers`
+for more information about this.
+
+Misc
+----
+
+Text aliasing
+~~~~~~~~~~~~~
+
+When convenient, avoid setting the ``opacity`` property on
+text as it will cause text to be aliased differently.
+
+HDPI support
+~~~~~~~~~~~~
+
+It's recommended to use SVG since it keeps the CSS clean when supporting
+multiple resolutions. See the :ref:`SVG Guidelines` for more information
+on SVG usage.
+
+However, if only 1x and 2x PNG assets are available, you can use this
+``@media`` query to target higher density displays (HDPI):
+
+.. code:: css
+
+   @media (min-resolution: 1.1dppx)
diff --git a/docs/code-quality/coding-style/format_cpp_code_with_clang-format.rst b/docs/code-quality/coding-style/format_cpp_code_with_clang-format.rst
new file mode 100644
index 0000000000..ca64835410
--- /dev/null
+++ b/docs/code-quality/coding-style/format_cpp_code_with_clang-format.rst
@@ -0,0 +1,272 @@
+=====================================
+Formatting C++ Code With clang-format
+=====================================
+
+Mozilla uses the Google coding style for whitespace, which is enforced
+using `clang-format <https://clang.llvm.org/docs/ClangFormat.html>`__. A
+specific version of the binary will be installed when
+``./mach clang-format`` or ``./mach bootstrap`` are run. We build our
+own binaries and update them as needed.
+
+Options are explicitly defined `in clang-format
+itself <https://github.com/llvm-mirror/clang/blob/e8a55f98df6bda77ee2eaa7f7247bd655f79ae0e/lib/Format/Format.cpp#L856>`__.
+If the options are changed in clang upstream, this might cause some
+changes in the Firefox tree. For this reason, it is best to use the
+mozilla-provided binaries.
+
+Manual formatting
+-----------------
+
+We provide a mach subcommand for running clang-format from the
+command-line. This wrapper handles ensuring the correct version of
+clang-format is installed and run.
+
+If clang-format isn’t installed, the binaries will be automatically
+downloaded from taskcluster and installed into ~/.mozbuild. We build our
+own clang-format binaries.
+
+
+Formatting local changes
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+   $ ./mach clang-format
+
+When run without arguments, it will run on a local diff. This could miss
+some reformatting (for example, when blocks are touched).
+(`searchfox <https://searchfox.org/mozilla-central/rev/501eb4718d73870892d28f31a99b46f4783efaa0/python/mozbuild/mozbuild/code-analysis/mach_commands.py#1620>`__)
+
+
+Formatting specific paths
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+   $ ./mach clang-format -p <path>     # Format <path> in-place
+   $ ./mach clang-format -p <path> -s  # Show changes
+
+The command also accepts a ``-p`` argument to reformat a specific
+directory or file, and a ``-s`` flag to show the changes instead of
+applying them to the working directory
+(`searchfox <https://searchfox.org/mozilla-central/rev/501eb4718d73870892d28f31a99b46f4783efaa0/python/mozbuild/mozbuild/code-analysis/mach_commands.py#1633>`__)
+
+
+Formatting specific commits / revisions
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+   $ ./mach clang-format -c HEAD # Format a single git commit
+   $ ./mach clang-format -c HEAD~~..HEAD # Format a range of git commits
+   $ ./mach clang-format -c . # Format a single mercurial revision
+
+The command accepts a ``-c`` argument that takes a revision number or
+commit range, and will format the lines modified by those commits.
+(`searchfox <https://searchfox.org/mozilla-central/rev/501eb4718d73870892d28f31a99b46f4783efaa0/python/mozbuild/mozbuild/code-analysis/mach_commands.py#1635>`__)
+
+
+Scripting Clang-Format
+~~~~~~~~~~~~~~~~~~~~~~
+
+Clang format expects that the path being passed to it is the path
+on-disk. If this is not the case, for example when formatting a
+temporary file, the "real" path must be specified. This can be done with
+the ``--assume-filename <path>`` argument.
+
+
+Configuring the clang-format commit hook
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+To run clang-format at commit phase, run ``mach boostrap`` or just add
+the following line in the ``hgrc`` file:
+
+.. code:: ini
+
+   [extensions]
+   clang-format = ~/.mozbuild/version-control-tools/hgext/clang-format
+
+We use a hg extension as they are more flexible than hooks.
+
+With git, the configuration is the following:
+
+::
+
+   # From the root git directory:
+   $ ln -s $(pwd)/tools/lint/hooks_clang_format.py .git/hooks/pre-commit
+
+You'll likely need to install the ``python-hglib`` package for your OS,
+or else you may get errors like ``abort: No module named hglib.client!``
+when you try to commit.
+
+
+Editor integration
+------------------
+
+It is possible to configure many editors to support running
+``clang-format`` automatically on save, or when run from within the
+editor.
+
+
+Editor plugins
+~~~~~~~~~~~~~~
+
+-  `Atom <https://atom.io/packages/clang-format>`__
+-  `BBEdit <http://clang.llvm.org/docs/ClangFormat.html#bbedit-integration>`__
+
+   -  `clang-format-bbedit.applescript <https://raw.githubusercontent.com/llvm-mirror/clang/master/tools/clang-format/clang-format-bbedit.applescript>`__
+
+-  Eclipse
+
+   -  Install the
+      `CppStyle <https://marketplace.eclipse.org/content/cppstyle>`__
+      plugin
+   -  In Preferences -> C/C++ -> CppStyle, set the clang-format path to
+      ~/.mozbuild/clang-tools/clang-tidy/bin/clang-format
+   -  (Optional) check "Run clang-format on file save"
+
+-  `Emacs <http://clang.llvm.org/docs/ClangFormat.html#emacs-integration>`__
+
+   -  `clang-format.el <https://raw.githubusercontent.com/llvm-mirror/clang/master/tools/clang-format/clang-format.el>`__
+      (Or install
+      `clang-format <http://melpa.org/#/clang-format>`__ from MELPA)
+   -  `google-c-style <http://melpa.org/#/google-c-style>`__ from MELPA
+
+-  `Sublime Text <https://packagecontrol.io/packages/Clang%20Format>`__
+
+   -  `alternative
+      tool <https://github.com/rosshemsley/SublimeClangFormat>`__
+
+-  `Vim <http://clang.llvm.org/docs/ClangFormat.html#vim-integration>`__
+
+   -  `clang-format.py <https://raw.githubusercontent.com/llvm-mirror/clang/master/tools/clang-format/clang-format.py>`__
+   -  `vim-clang-format <https://github.com/rhysd/vim-clang-format>`__
+
+-  `Visual
+   Studio <https://marketplace.visualstudio.com/items?itemName=LLVMExtensions.ClangFormat>`__
+
+   -  `llvm.org plugin <http://llvm.org/builds/>`__
+   -  `Integrated support in Visual Studio
+      2017 <https://blogs.msdn.microsoft.com/vcblog/2018/03/13/clangformat-support-in-visual-studio-2017-15-7-preview-1/>`__
+
+-  `Visual Studio
+   Code <https://marketplace.visualstudio.com/items?itemName=xaver.clang-format>`__
+-  `XCode <https://github.com/travisjeffery/ClangFormat-Xcode>`__
+-  `Script for patch
+   reformatting <http://clang.llvm.org/docs/ClangFormat.html#script-for-patch-reformatting>`__
+
+   -  `clang-format-diff.py <https://raw.githubusercontent.com/llvm-mirror/clang/master/tools/clang-format/clang-format-diff.py>`__
+
+
+Configuration
+~~~~~~~~~~~~~
+
+These tools generally run clang-format themselves, and won't use
+``./mach clang-format``. The binary installed by our tooling will be
+located at ``~/.mozbuild/clang-tools/clang-tidy/bin/clang-format``.
+
+You typically shouldn't need to specify any other special configuration
+in your editor besides the clang-format binary. Most of the
+configuration that clang-format relies on for formatting is stored
+inside our source tree. More specifically, using the .clang-format file
+located in the root of the repository. Please note that this doesn't
+include the list of ignored files and directories (provided by
+.clang-format-ignore which is a feature provided by the mach command
+wrapper).
+
+Coding style configuration is done within clang-format itself. When we
+change the configuration (incorrect configuration, new feature in clang,
+etc), we use `local
+overrides <https://searchfox.org/mozilla-central/rev/501eb4718d73870892d28f31a99b46f4783efaa0/.clang-format>`__.
+
+
+Ignored files & directories
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+We maintain a `list of ignored directories and
+files <https://searchfox.org/mozilla-central/rev/501eb4718d73870892d28f31a99b46f4783efaa0/.clang-format-ignore>`__,
+which is used by ``./mach clang-format``. This is generally only used
+for code broken by clang-format, and third-party code.
+
+
+Ignored code hunks
+~~~~~~~~~~~~~~~~~~
+
+Sections of code may have formatting disabled using comments. If a
+section must not be formatted, the following comments will disable the
+reformat:
+
+::
+
+   // clang-format off
+   my code which should not be reformatted
+   // clang-format on
+
+You can find an `example of code not
+formatted <https://searchfox.org/mozilla-central/rev/501eb4718d73870892d28f31a99b46f4783efaa0/xpcom/io/nsEscape.cpp#22>`__.
+
+
+Merging formatted and unformatted code
+--------------------------------------
+
+During the transition to using chromium style enforced by clang-format
+for all code in tree, it will often be necessary to rebase non-formatted
+code onto a formatted tree.
+
+
+Mercurial
+~~~~~~~~~
+
+The ``format-source`` extension, now bundled with
+``version-control-tools``, and installed by ``./mach bootstrap``, may be
+used to seamlessly handle this situation. More details may be found in
+this
+`document <https://docs.google.com/document/d/13AwAsvKMhH0mflDlfatBqn6LmZHiQih76oxM4zfrPl4/edit>`__.
+
+The parent changeset of the reformat has been tagged as
+``PRE_TREEWIDE_CLANG_FORMAT``.
+
+
+Git
+~~~
+
+To perform a rebase onto mozilla-central after the merge, a handy merge
+driver, ``clang-format-merge``, has been written:
+
+.. code:: shell
+
+   $ git clone https://github.com/emilio/clang-format-merge
+   $ /path/to/clang-format-merge/git-wrapper rebase <upstream>
+
+The wrapper should clean up after itself, and the clone may be deleted
+after the rebase is complete.
+
+
+Ignore lists
+------------
+
+To make sure that the blame/annotate features of Mercurial or git aren't
+affected. Two files are maintained to keep track of the reformatting
+commits.
+
+
+With Mercurial
+~~~~~~~~~~~~~~
+
+| The list is stored in
+  `https://searchfox.org/mozilla-central/source/.hg-annotate-ignore-revs </en-US/docs/>`__
+| Commit messages should also contain the string ``# ignore-this-changeset``
+
+The syntax in this file is generated using the following syntax:
+
+::
+
+   $ hg log --template '{node} - {author|person} - {desc|strip|firstline}\n'
+
+With git
+~~~~~~~~
+
+The list is stored in
+`https://searchfox.org/mozilla-central/source/.git-blame-ignore-revs </en-US/docs/>`__
+and contains git revisions for both gecko-dev and the git cinnabar
+repository.
diff --git a/docs/code-quality/coding-style/index.rst b/docs/code-quality/coding-style/index.rst
new file mode 100644
index 0000000000..e62ce910ca
--- /dev/null
+++ b/docs/code-quality/coding-style/index.rst
@@ -0,0 +1,20 @@
+Coding style
+============
+
+Firefox code is using different programming languages.
+For each language, we are enforcing a specific coding style.
+
+Getting Help
+------------
+
+If you need help or have questions, please don’t hesitate to contact us via Matrix
+in the "Lint and Formatting" room
+(`#lint:mozilla.org <https://chat.mozilla.org/#/room/#lint:mozilla.org>`_).
+
+
+.. toctree::
+  :caption: Coding Style User Guide
+  :maxdepth: 2
+  :glob:
+
+  *
diff --git a/docs/code-quality/coding-style/rtl_guidelines.rst b/docs/code-quality/coding-style/rtl_guidelines.rst
new file mode 100644
index 0000000000..8b2a6bcf04
--- /dev/null
+++ b/docs/code-quality/coding-style/rtl_guidelines.rst
@@ -0,0 +1,356 @@
+RTL Guidelines
+==============
+
+RTL languages such as Arabic, Hebrew, Persian and Urdu are read and
+written from right-to-left, and the user interface for these languages
+should be mirrored to ensure the content is easy to understand.
+
+When a UI is changed from LTR to RTL (or vice-versa), it’s often called
+mirroring. An RTL layout is the mirror image of an LTR layout, and it
+affects layout, text, and graphics.
+
+In RTL, anything that relates to time should be depicted as moving from
+right to left. For example, forward points to the left, and backwards
+points to the right.
+
+Mirroring layout
+~~~~~~~~~~~~~~~~
+
+When a UI is mirrored, these changes occur:
+
+-  Text fields icons are displayed on the opposite side of a field
+-  Navigation buttons are displayed in reverse order
+-  Icons that communicate direction, like arrows, are mirrored
+-  Text is usually aligned to the right
+
+In CSS, while it's possible to apply a rule for LTR and a separate one
+specifically for RTL, it's usually better to use CSS `Logical Properties <https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Logical_Properties>`_
+which provide the ability to control layout through logical, rather than
+physical mappings.
+
++---------------------------------------------------------+--------------------------------------------------+
+| Do                                                      | Don't do                                         |
++---------------------------------------------------------+--------------------------------------------------+
+| ``margin-inline-start: 5px``                            | ``margin-left: 5px``                             |
++---------------------------------------------------------+--------------------------------------------------+
+| ``padding-inline-end: 5px``                             | ``padding-right: 5px``                           |
++---------------------------------------------------------+--------------------------------------------------+
+| ``float: inline-start``                                 | ``float: left``                                  |
++---------------------------------------------------------+--------------------------------------------------+
+| ``inset-inline-start: 5px``                             | ``left: 5px``                                    |
++---------------------------------------------------------+--------------------------------------------------+
+| ``border-inline-end: 1px``                              | ``border-right: 1px``                            |
++---------------------------------------------------------+--------------------------------------------------+
+| ``border-{start/end}-{start/end}-radius: 2px``          | ``border-{top/bottom}-{left/right}-radius: 2px`` |
++---------------------------------------------------------+--------------------------------------------------+
+| ``padding: 1px 2px``                                    | ``padding: 1px 2px 1px 2px``                     |
++---------------------------------------------------------+--------------------------------------------------+
+| ``margin-block: 1px 3px`` && ``margin-inline: 4px 2px`` | ``margin: 1px 2px 3px 4px``                      |
++---------------------------------------------------------+--------------------------------------------------+
+| ``text-align: start`` or ``text-align: match-parent``   | ``text-align: left``                             |
+| (depends on the context)                                |                                                  |
++---------------------------------------------------------+--------------------------------------------------+
+
+When there is no special RTL-aware property available, or when
+left/right properties must be used specifically for RTL, use the pseudo
+``:-moz-locale-dir(rtl)`` (for XUL documents) or ``:dir(rtl)`` (for HTML
+documents).
+
+For example, this rule covers LTR to display searchicon.svg 7 pixels
+from the left:
+
+.. code:: css
+
+   .search-box {
+     background-image: url(chrome://path/to/searchicon.svg);
+     background-position: 7px center;
+   }
+
+but an additional rule is necessary to cover RTL and place the search
+icon on the right:
+
+.. code:: css
+
+   .search-box:dir(rtl) {
+     background-position-x: right 7px;
+   }
+
+.. warning::
+
+   It may be inappropriate to use logical properties when embedding LTR
+   within RTL contexts. This is described further in the document.
+
+Mirroring elements
+~~~~~~~~~~~~~~~~~~
+
+RTL content also affects the direction in which some icons and images
+are displayed, particularly those depicting a sequence of events.
+
+What to mirror
+^^^^^^^^^^^^^^
+
+-  Icons or animations that imply directionality or motion like
+   back/forward buttons or progress bars
+-  Icons that imply text direction, like
+   `reader-mode.svg <https://searchfox.org/mozilla-central/rev/f9beb753a84aa297713d1565dcd0c5e3c66e4174/browser/themes/shared/icons/reader-mode.svg>`__
+-  Icons that imply location of UI elements in the screen, like
+   `sidebars-right.svg <https://searchfox.org/mozilla-central/rev/74cc0f4dce444fe0757e2a6b8307d19e4d0e0212/browser/themes/shared/icons/sidebars-right.svg>`__,
+   `open-in-new.svg <https://searchfox.org/mozilla-central/rev/f9beb753a84aa297713d1565dcd0c5e3c66e4174/toolkit/themes/shared/icons/open-in-new.svg>`__,
+   `default theme's preview.svg <https://searchfox.org/mozilla-central/rev/f9beb753a84aa297713d1565dcd0c5e3c66e4174/toolkit/mozapps/extensions/default-theme/preview.svg>`__
+   or
+   `pane-collapse.svg <https://searchfox.org/mozilla-central/rev/74cc0f4dce444fe0757e2a6b8307d19e4d0e0212/devtools/client/debugger/images/pane-collapse.svg>`__
+-  Icons representing objects that are meant to be handheld should look
+   like they're being right-handed, like the `magnifying glass
+   icon <https://searchfox.org/mozilla-central/rev/e7c61f4a68b974d5fecd216dc7407b631a24eb8f/toolkit/themes/windows/global/icons/search-textbox.svg>`__
+-  Twisties in their collapsed state. Note that if the context in which
+   they appear is LTR (e.g. code in a devtools HTML view), they should
+   not be mirrored, even if the user might be using an RTL locale.
+
+What NOT to mirror
+^^^^^^^^^^^^^^^^^^
+
+-  Text/numbers
+-  Icons containing text/numbers
+-  Icons/animations that are direction neutral
+-  Icons that wouldn't look differently if they'd be mirrored, like `X
+   buttons <https://searchfox.org/mozilla-central/rev/a78233c11a6baf2c308fbed17eb16c6e57b6a2ac/devtools/client/debugger/images/close.svg>`__
+   or the `bookmark
+   star <https://searchfox.org/mozilla-central/rev/a78233c11a6baf2c308fbed17eb16c6e57b6a2ac/browser/themes/shared/icons/bookmark-hollow.svg>`__
+   icon, or any other symmetric icon
+-  Icons that should look the same as LTR, like icons related to code
+   (which is always LTR) like
+   `tool-webconsole.svg <https://searchfox.org/mozilla-central/rev/74cc0f4dce444fe0757e2a6b8307d19e4d0e0212/devtools/client/themes/images/tool-webconsole.svg>`__
+-  Checkmark icons
+-  Video/audio player controls
+-  Product logos
+-  Order of size dimensions (e.g., ``1920x1080`` should not become
+   ``1080x1920``)
+-  Order of size units (e.g., ``10 px`` should not become ``px 10``
+   (unless the size unit is localizable))
+
+How
+^^^
+
+The most common way to mirror images is by flipping the X axis:
+
+.. code:: css
+
+   transform: scaleX(-1);
+
+Or, if you're already using ``transform`` with a different value on the same
+element, you can also use `scale`:
+
+.. code:: css
+
+   scale: -1 1;
+
+Note that mirroring images that way doesn't work when the image is a part of
+an element with text using ``background-image``, because then the text would
+be mirrored along with the image, and the image would be positioned incorrectly.
+For such cases, try to use a different method for displaying the image,
+like having it as an element all on its own.
+If that's not possible, add a separate pre-mirrored image asset and specify
+it in a separate ``:dir(rtl)`` rule:
+
+.. code:: css
+
+   .element-with-icon {
+     background-image: url("path/to/image/image.svg");
+   }
+
+   .element-with-icon:dir(rtl) {
+     background-image: url("path/to/image/image-rtl.svg");
+   }
+
+For animations like a progress bar, when using ``@keyframes`` to change
+the ``transform: translateX()`` states, make sure to add a different
+``@keyframes`` suited for RTL, and target that in a separate ``:dir()`` rule:
+
+.. code:: css
+
+   #progressbar {
+     animation: progressbar-animation 1s linear infinite;
+   }
+
+   #progressbar:dir(rtl) {
+     animation-name: progressbar-animation-rtl;
+   }
+
+   @keyframes progressbar-animation {
+     0% {
+       transform: translateX(-100px);
+     }
+     100% {
+       transform: translateX(0);
+     }
+   }
+
+   @keyframes progressbar-animation-rtl {
+     0% {
+       transform: translateX(100px);
+     }
+     100% {
+       transform: translateX(0);
+     }
+   }
+
+Likewise, if you're using ``transform-origin``, make sure to specify the
+correct origin for RTL:
+
+.. code:: css
+
+   #progressbar {
+     transform-origin: 0 0;
+   }
+
+   #progressbar:dir(rtl) {
+     transform-origin: 100% 0;
+   }
+
+LTR text inside RTL contexts
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+By default, in RTL locales, some symbols like ``/`` and ``.`` will be moved
+around and won't be displayed in the order that they were typed in. This
+may be problematic for URLs for instance, where you don't want dots to
+change position.
+
+Here's a non-exhaustive list of elements that should be displayed like
+they would be in LTR locales:
+
+-  Paths (e.g., C:\\Users\\username\\Desktop)
+-  Full URLs
+-  Code and code containers (like the DevTools' Inspector or the CSS
+   rules panel)
+-  about:config preference names and values
+-  Telephone numbers
+-  Usernames & passwords (most sites on the web expect LTR
+   usernames/passwords, but there may be exceptions)
+-  Other text fields where only LTR text is expected
+
+To make sure these are displayed correctly, you can use one of the
+following on the relevant element:
+
+-  ``direction: ltr``
+-  ``dir="ltr"`` in HTML
+
+Since the direction of such elements is forced to LTR, the text will
+also be aligned to the left, which is undesirable from an UI
+perspective, given that is inconsistent with the rest of the RTL UI
+which has text usually aligned to the right. You can fix this using
+``text-align: match-parent``. In the following screenshot, both text
+fields (username and password) and the URL have their direction set to
+LTR (to display text correctly), but the text itself is aligned to the
+right for consistency with the rest of the UI:
+
+.. image:: about-logins-rtl.png
+   :alt: about:logins textboxes in RTL layout
+
+However, since the direction in LTR, this also means that the start/end
+properties will correspond to left/right respectively, which is probably
+not what you expect. This means you have to use extra rules instead of
+using logical properties.
+
+Here's a full code example:
+
+.. code:: css
+
+   .url {
+     direction: ltr; /* Force text direction to be LTR */
+
+     /* `start` (the default value) will correspond to `left`,
+      * so we match the parent's direction in order to align the text to the right */
+     text-align: match-parent;
+   }
+
+   /* :dir(ltr/rtl) isn't meaningful on .url, since it has direction: ltr, hence
+    * why it is matched on .container. */
+   .container:dir(ltr) .url {
+     padding-left: 1em;
+   }
+
+   .container:dir(rtl) .url {
+     padding-right: 1em;
+   }
+
+.. note::
+
+   The LTR rule is separate from the global rule to avoid having the
+   left padding apply on RTL without having to reset it in the RTL rule.
+
+Auto-directionality
+^^^^^^^^^^^^^^^^^^^
+
+Sometimes, the text direction on an element should vary dynamically
+depending on the situation. This can be the case for a search input for
+instance, a user may input a query in an LTR language, but may also
+input a query in a RTL language. In this case, the search input has to
+dynamically pick the correct directionality based on the first word, in
+order to display the query text correctly. The typical way to do this is
+to use ``dir="auto"`` in HTML. It is essential that
+``text-align: match-parent`` is set, to avoid having the text alignment
+change based on the query, and logical properties also cannot be used on
+the element itself given they can change meaning depending on the query.
+
+Testing
+~~~~~~~
+
+To test for RTL layouts in Firefox, you can go to about:config and
+set ``intl.l10n.pseudo`` to ``bidi``, or select the ``Enable "bidi" locale``
+option in the 3-dots menu in the :doc:`Browser Toolbox </devtools-user/browser_toolbox/index>`.
+The Firefox UI should immediately flip, but a restart may be required
+to take effect in some Firefox features and interactions.
+
+.. note::
+
+   When testing with ``intl.l10n.pseudo`` set to ``bidi``, you may see some
+   oddities regarding text ordering due to the nature of displaying LTR
+   text in RTL layout.
+
+   .. image:: about-protections-rtl.png
+      :alt: about:protections in RTL layout- English vs. Hebrew
+
+   This shouldn't be an issue when using an actual RTL build or language pack.
+
+How to spot RTL-related issues
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+-  Punctuation marks should appear on the left side of a
+   word/sentence/paragraph on RTL, so if a *localizable* string appears
+   in the UI with a dot, colon, ellipsis, question or exclamation mark
+   on the right side of the text, this probably means that the text
+   field is forced to be displayed as LTR.
+-  If icons/images/checkmarks do not appear on the opposite side of
+   text, when compared to LTR.
+-  If buttons (like the close button, "OK" and "Cancel" etc.) do not
+   appear on the opposite side of the UI and not in the opposite order,
+   when compared to LTR.
+-  If paddings/margins/borders are not the same from the opposite side,
+   when compared to LTR.
+-  Although Hebrew uses ``1 2 3``, all the other RTL locales we support
+   should use ``١ ٢ ٣`` as digits. So if you see ``1 2 3`` on any such
+   locale, that likely indicates a bug.
+-  If navigating in the UI using the left/right arrow keys does not
+   select the correct element (i.e., pressing Left selects an item on
+   the right).
+-  If navigating in the UI using the Tab key does not focus elements
+   from right to left, in an RTL context.
+-  If code is displayed as RTL (e.g., ``;padding: 20px`` - the semicolon
+   should appear on the right side of the code). Code can still be
+   aligned to the right if it appears in an RTL context.
+
+See also
+~~~~~~~~
+
+-  `RTL Best
+   Practices <https://docs.google.com/document/d/1Rc8rvwsLI06xArFQouTinSh3wNte9Sqn9KWi1r7xY4Y/edit#heading=h.pw54h41h12ct>`__
+-  Building RTL-Aware Web Apps & Websites: `Part
+   1 <https://hacks.mozilla.org/2015/09/building-rtl-aware-web-apps-and-websites-part-1/>`__,
+   `Part
+   2 <https://hacks.mozilla.org/2015/10/building-rtl-aware-web-apps-websites-part-2/>`__
+
+Credits
+~~~~~~~
+
+Google's `Material Design guide for
+RTL <https://material.io/design/usability/bidirectionality.html>`__
diff --git a/docs/code-quality/coding-style/svg_guidelines.rst b/docs/code-quality/coding-style/svg_guidelines.rst
new file mode 100644
index 0000000000..f34317c91a
--- /dev/null
+++ b/docs/code-quality/coding-style/svg_guidelines.rst
@@ -0,0 +1,347 @@
+SVG Guidelines
+==============
+
+Pros and cons of SVG for images
+-------------------------------
+
+When used as a document format there is usually a compelling reason that
+makes SVG the only solution. When used as an `image
+format <https://developer.mozilla.org/en-US/docs/Web/SVG/SVG_as_an_Image>`__,
+it is sometimes less obvious whether it would be best to use SVG or a
+raster image format for any given image. The vector format SVG and
+raster formats like PNG both have their place. When choosing whether or
+not to use SVG it is best to understand the advantages and disadvantages
+of both.
+
+File size
+   Whether SVG or a raster format will produce a smaller file for a
+   given image depends very much on the image. For example, consider an
+   image of a path with a gradient fill. The size of an SVG of this
+   image will be the same regardless of the dimensions of the image. On
+   the other hand the size of a raster file of the same image will
+   likely vary tremendously depending on the dimensions of the image
+   since the larger the dimensions the more pixel data the file needs to
+   store. At very small dimensions (the extreme case being 1px x 1px)
+   the raster file will likely be much smaller than the SVG file since
+   it only needs to store one pixel of data. At large dimensions the
+   raster file may be much larger than the SVG file.
+Scalability, with caveats
+   One of the primary advantages of SVG is that as it is scaled it does
+   not pixelate. However, this is not to say that it always does away
+   with the need to have a collection of raster images for display at
+   different scales. This can be particularly true for icons. While SVG
+   may scale well enough for flat-ish icons without a lot of detail, for
+   icons that try to pack in a lot of detail graphic artists generally
+   `want to be able to pixel
+   tweak <https://www.pushing-pixels.org/2011/11/04/about-those-vector-icons.html>`__.
+Performance
+   While SVG provides a lot of flexibility in terms of scaling,
+   themability, etc. this flexibility depends on doing computations for
+   SVG images at the time they're displayed, rather than at the time the
+   author creates them. Consider an image that involves some complex
+   gradients and filters. If saved as a raster image then the work to
+   rasterize the gradients and filters takes place on the authors
+   computer before the result is stored in the raster file. This work
+   doesn't need to be redone when the image is displayed on someone
+   else's computer. On the other hand, if the image is saved as an SVG
+   image then all this work needs to be done each time the SVG is
+   displayed on someone else's computer. This isn't to say that SVG
+   images are always slower than raster equivalents. In fact it can be
+   faster to send vector information from an SVG to a user's GPU than it
+   is to extract raster data from an equivalent raster image. And even
+   when an SVG image is slower than a raster equivalent, the difference
+   is usually not noticeable. However, just don't fall into the trap of
+   thinking that SVGs are faster than equivalent raster images, or vice
+   versa. Once again, "it depends".
+
+Authoring guidelines
+--------------------
+
+A lot of SVG files (particularly those generated by SVG editors) ship
+without being cleaned up and can contain a ton of junk that bloats the
+file size and slows down rendering. In general the best way to combat
+this is to first run SVG files through a linter such as
+`svgo <https://github.com/svg/svgo>`__ (see the Tools section below).
+However, when authoring SVGs by hand here are some best practices to
+help keep them lightweight. These rules are based on some real examples
+seen in Mozilla's code.
+
+Basics
+~~~~~~
+
+-  Two spaces indenting
+-  No useless whitespaces or line breaks (see below for more details)
+-  Adding a license header
+-  Use double quotes
+
+Whitespace and line breaks
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Whitespace
+^^^^^^^^^^
+
+In addition to trailing whitespace at the end of lines, there are a few
+more cases more specific to SVGs:
+
+-  Trailing whitespaces in attribute values (usually seen in path
+   definitions)
+-  Excessive whitespace in path or polygon points definition
+
+Whitespace examples
+^^^^^^^^^^^^^^^^^^^
+
+This path:
+
+.. code:: html
+
+   <path d=" M5,5    L1,1z ">
+
+can be cut down to this:
+
+.. code:: html
+
+   <path d="M5,5 L1,1z">
+
+Similarly, this polygon:
+
+.. code:: html
+
+   <polygon points="  0,0   4,4  4,0  "/>
+
+can be cut down to this:
+
+.. code:: html
+
+   <polygon points="0,0 4,4 4,0"/>
+
+Line breaks
+^^^^^^^^^^^
+
+You should only use line breaks for logical separation or if they help
+make the file readable. You should avoid line breaks between every
+single element or within attribute values. It's recommended to put the
+attributes on the same line as their tag names, if possible. You should
+also put the shortest attributes first, so they are easier to spot.
+
+Unused tags and attributes
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Editor metadata
+^^^^^^^^^^^^^^^
+
+Vector editors (Inkscape, Adobe Illustrator, Sketch) usually add a bunch
+of metadata in SVG files while saving them. Metadata can mean many
+things, including:
+
+-  The typical "Created with *editor*" comments
+-  Non-standard editor specific tags and attributes (``sketch:foo``,
+   ``illustrator:foo``, ``sopodi:foo``, …)
+-  The `XML
+   namespace <https://developer.mozilla.org/en-US/docs/Web/SVG/Namespaces_Crash_Course>`__
+   definition that comes with the latter (``xmlns:sketch``,
+   ``xmlns:sopodi``, …)
+
+Other metadata
+^^^^^^^^^^^^^^
+
+In addition to non-standard editor metadata, standard compliant metadata
+also exists. Typical examples of this are ``<title>`` and ``<desc>``
+tags. Although this kind of data is supported by the browser, it can
+only be displayed when the SVG is opened in a new tab. Plus, in most of
+the cases, the filename is quite descriptive So it's recommended to
+remove that kind of metadata since it doesn't bring much value.
+
+You shouldn't include DOCTYPEs in your SVGs either; they are a source of
+many issues, and the SVG WG recommends not to include them. See `SVG
+Authoring
+guidelines <https://jwatt.org/svg/authoring/#doctype-declaration>`__.
+
+Avoid the use of CDATA sections
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+`CDATA
+sections <https://developer.mozilla.org/en-US/docs/Web/API/CDATASection>`__
+are used to avoid parsing some text as HTML. Most of time, CDATA isn't
+needed, for example, the content in ``<style>`` tags doesn't need to be
+wrapped in a CDATA section as the content inside the tag is already
+correctly parsed as CSS.
+
+Invisible shapes
+^^^^^^^^^^^^^^^^
+
+There are two kinds of invisible shapes: The off-screen ones and the
+uncolored ones.
+
+The offscreen shapes are hard to spot, even with an automated tool, and
+are usually context aware. Those kinds of shapes are visible but off the
+`SVG view
+box <https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/viewBox>`__.
+Here's `an
+example <https://hg.mozilla.org/mozilla-central/diff/9fb143f3b36a/browser/themes/shared/heartbeat-star-lit.svg>`__
+of a file with offscreen shapes.
+
+On the other hand, the uncolored ones are easier to spot, since they
+usually come with styles making them invisible. They must meet two
+conditions: they must be devoid of any fill (or a transparent one) or
+stroke.
+
+Unused attributes on root ``<svg>`` element
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The root ``<svg>`` element can also host many useless attributes. Here's
+an
+`example <https://hg.mozilla.org/mozilla-central/diff/2d38fecce226/browser/components/loop/content/shared/img/icons-10x10.svg>`__
+taking into account the list below:
+
+-  ``version``
+-  ``x="0"`` and ``y="0"``
+-  ``enable-background`` (unsupported by Gecko and now deprecated by the
+   Filter Effects specification)
+-  ``id`` (id on root element has no effect)
+-  ``xmlns:xlink`` attribute when there are no ``xlink:href`` attributes
+   used throughout the file
+-  Other unused `XML
+   Namespace <https://developer.mozilla.org/en-US/docs/Web/SVG/Namespaces_Crash_Course>`__
+   definitions
+-  ``xml:space`` when there is no text used in the file
+
+Other
+^^^^^
+
+-  Empty tags, this may be obvious, but those are sometimes found in
+   SVGs
+-  Unreferenced ids (usually on gradient stops, but also on shapes or
+   paths)
+-  ``clip-rule`` attribute when the element *is not* a descendant of a
+   ``<clipPath>``
+-  ``fill-rule`` attribute when the element *is* a descendant of a
+   ``<clipPath>``
+-  Unreferenced/Unused clip paths, masks or defs
+   (`example <https://hg.mozilla.org/mozilla-central/diff/2d38fecce226/toolkit/themes/shared/reader/RM-Plus-24x24.svg>`__)
+
+Styling
+~~~~~~~
+
+Styling basics
+^^^^^^^^^^^^^^
+
+-  Privilege short lowercase hex for colors
+-  Don't use excessive precision for numeric values (usually comes from
+   illustrator)
+-  Use descriptive IDs
+-  Avoid inline styles and use class names or SVG attributes
+
+Styling examples
+''''''''''''''''
+
+Here are some examples for excessive number precision:
+
+-  5.000000e-02 → 0.05 (as seen
+   `here <https://hg.mozilla.org/mozilla-central/diff/2d38fecce226/browser/themes/shared/devtools/images/tool-network.svg#l1.31>`__)
+-  -3.728928e-10 → 0 (as seen
+   `here <https://hg.mozilla.org/mozilla-central/diff/2d38fecce226/browser/themes/shared/aboutNetError_alert.svg#l1.12>`__)
+-  translate(0.000000, -1.000000) → translate(0, -1) (as seen
+   `here <https://hg.mozilla.org/mozilla-central/diff/2d38fecce226/browser/themes/shared/heartbeat-icon.svg#l1.13>`__)
+
+As for descriptive IDs:
+
+-  For gradients: SVG_ID1 → gradient1 (as seen
+   `here <https://hg.mozilla.org/mozilla-central/diff/2d38fecce226/browser/themes/shared/aboutNetError_alert.svg#l1.12>`__)
+
+Use of class names
+^^^^^^^^^^^^^^^^^^
+
+-  Avoid using a class if that class is only used once in the file
+-  If that class only sets a fill or a stroke, it's better to set the
+   fill/stroke directly on the actual shape, instead of introducing a
+   class just for that shape. You can also use SVG grouping to avoid
+   duplicating those attributes
+-  Avoid introducing variants of the same file (color/style variants),
+   and use sprites instead (with class names)
+
+Default style values
+^^^^^^^^^^^^^^^^^^^^
+
+There's usually no need to set the default style value unless you're
+overriding a style. Here are some commonly seen examples:
+
+-  ``style="display: none;"`` on ``<defs>`` elements (a ``<defs>``
+   element is hidden by default)
+-  ``type="text/css"`` on ``<style>`` elements
+-  ``stroke: none`` or ``stroke-width: 0``
+
+SVG grouping
+~~~~~~~~~~~~
+
+Style grouping
+^^^^^^^^^^^^^^
+
+Group similarly styled shapes under one ``<g>`` tag; this avoids having
+to set the same class/styles on many shapes.
+
+Avoid excessive grouping
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+Editors can sometimes do excessive grouping while exporting SVGs. This
+is due to the way editors work.
+
+Nested groups
+'''''''''''''
+
+Avoid multiple-level nesting of groups, these make the SVG less
+readable.
+
+Nested transforms
+'''''''''''''''''
+
+Some editors use ``<g>`` tags to do nested transforms, which is usually
+not needed. You can avoid this by doing basic algebra, for example:
+
+.. code:: xml
+
+   <g transform="translate(-62, -310)"><shape transform="translate(60, 308)"/></g>
+
+can be cut down to:
+
+.. code:: xml
+
+   <shape transform="translate(-2,-2)"/>
+
+because: -62+60 = -310+308 = -2
+
+Performance tips
+~~~~~~~~~~~~~~~~
+
+These rules are optional, but they help speeding up the SVG.
+
+-  Avoid using a ``<use>`` tag when that ``<use>`` tag is being
+   referenced only once in the whole file.
+-  Instead of using CSS/SVG
+   `transforms <https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/transform>`__,
+   apply directly the transform on the path/shape definition.
+
+Tools
+~~~~~
+
+Tools can help to clean SVG files. Note, however that some of the rules
+stated above can be hard to detect with automated tools since they
+require too much context-awareness. To this date, there doesn't seem to
+be a tool that handles all of the above. However, there are some
+utilities that cover parts of this document:
+
+-  Mostly complete command line tool: https://github.com/svg/svgo
+-  Alternatives to SVGO:
+
+   -  https://github.com/RazrFalcon/svgcleaner
+   -  https://github.com/scour-project/scour
+
+-  GUI for command line tool (use with "Prettify code" and "Remove
+   ``<title>``" options on): https://jakearchibald.github.io/svgomg/
+-  Good alternative to SVGO/SVGOMG:
+   https://petercollingridge.appspot.com/svg-editor
+-  Fixes the excessive number precision:
+   https://simon.html5.org/tools/js/svg-optimizer/
+-  Converts inline styles to SVG
+   attributes: https://www.w3.org/wiki/SvgTidy
+-  RaphaelJS has a couple of utilities that may be useful:
+   `raphael.js <https://dmitrybaranovskiy.github.io/raphael/>`__
diff --git a/docs/code-quality/coding-style/using_cxx_in_firefox_code.rst b/docs/code-quality/coding-style/using_cxx_in_firefox_code.rst
new file mode 100644
index 0000000000..92decbdf1e
--- /dev/null
+++ b/docs/code-quality/coding-style/using_cxx_in_firefox_code.rst
@@ -0,0 +1,1075 @@
+Using C++ in Mozilla code
+=========================
+
+C++ language features
+---------------------
+
+Mozilla code only uses a subset of C++. Runtime type information (RTTI)
+is disabled, as it tends to cause a very large increase in codesize.
+This means that ``dynamic_cast``, ``typeid()`` and ``<typeinfo>`` cannot
+be used in Mozilla code. Also disabled are exceptions; do not use
+``try``/``catch`` or throw any exceptions. Libraries that throw
+exceptions may be used if you are willing to have the throw instead be
+treated as an abort.
+
+On the side of extending C++, we compile with ``-fno-strict-aliasing``.
+This means that when reinterpreting a pointer as a differently-typed
+pointer, you don't need to adhere to the "effective type" (of the
+pointee) rule from the standard (aka. "the strict aliasing rule") when
+dereferencing the reinterpreted pointer. You still need make sure that
+you don't violate alignment requirements and need to make sure that the
+data at the memory location pointed to forms a valid value when
+interpreted according to the type of the pointer when dereferencing the
+pointer for reading. Likewise, if you write by dereferencing the
+reinterpreted pointer and the originally-typed pointer might still be
+dereferenced for reading, you need to make sure that the values you
+write are valid according to the original type. This value validity
+issue is moot for e.g. primitive integers for which all bit patterns of
+their size are valid values.
+
+-  As of Mozilla 59, C++14 mode is required to build Mozilla.
+-  As of Mozilla 67, MSVC can no longer be used to build Mozilla.
+-  As of Mozilla 73, C++17 mode is required to build Mozilla.
+
+This means that C++17 can be used where supported on all platforms. The
+list of acceptable features is given below:
+
+.. list-table::
+   :widths: 25 25 25 25
+   :header-rows: 3
+
+   * -
+     - GCC
+     - Clang
+     -
+   * - Current minimal requirement
+     - 8.1
+     - 7.0
+     -
+   * - Feature
+     - GCC
+     - Clang
+     - Can be used in code
+   * - ``type_t &&``
+     - 4.3
+     - 2.9
+     - Yes (see notes)
+   * - ref qualifiers on methods
+     - 4.8.1
+     - 2.9
+     - Yes
+   * - default member-initializers (except for bit-fields)
+     - 4.7
+     - 3.0
+     - Yes
+   * - default member-initializers (for bit-fields)
+     - 8
+     - 6
+     - **No**
+   * - variadic templates
+     - 4.3
+     - 2.9
+     - Yes
+   * - Initializer lists
+     - 4.4
+     - 3.1
+     - Yes
+   * - ``static_assert``
+     - 4.3
+     - 2.9
+     - Yes
+   * - ``auto``
+     - 4.4
+     - 2.9
+     - Yes
+   * - lambdas
+     - 4.5
+     - 3.1
+     - Yes
+   * - ``decltype``
+     - 4.3
+     - 2.9
+     - Yes
+   * - ``Foo<Bar<T>>``
+     - 4.3
+     - 2.9
+     - Yes
+   * - ``auto func() -> int``
+     - 4.4
+     - 3.1
+     - Yes
+   * - Templated aliasing
+     - 4.7
+     - 3.0
+     - Yes
+   * - ``nullptr``
+     - 4.6
+     - 3.0
+     - Yes
+   * - ``enum foo : int16_t`` {};
+     - 4.4
+     - 2.9
+     - Yes
+   * - ``enum class foo {}``;
+     - 4.4
+     - 2.9
+     - Yes
+   * - ``enum foo;``
+     - 4.6
+     - 3.1
+     - Yes
+   * - ``[[attributes]]``
+     - 4.8
+     - 3.3
+     - **No** (see notes)
+   * - ``constexpr``
+     - 4.6
+     - 3.1
+     - Yes
+   * - ``alignas``
+     - 4.8
+     - 3.3
+     - Yes
+   * - ``alignof``
+     - 4.8
+     - 3.3
+     - Yes, but see notes ; only clang 3.6 claims as_feature(cxx_alignof)
+   * - Delegated constructors
+     - 4.7
+     - 3.0
+     - Yes
+   * - Inherited constructors
+     - 4.8
+     - 3.3
+     - Yes
+   * - ``explicit operator bool()``
+     - 4.5
+     - 3.0
+     - Yes
+   * - ``char16_t/u"string"``
+     - 4.4
+     - 3.0
+     - Yes
+   * - ``R"(string)"``
+     - 4.5
+     - 3.0
+     - Yes
+   * - ``operator""()``
+     - 4.7
+     - 3.1
+     - Yes
+   * - ``=delete``
+     - 4.4
+     - 2.9
+     - Yes
+   * - ``=default``
+     - 4.4
+     - 3.0
+     - Yes
+   * - unrestricted unions
+     - 4.6
+     - 3.1
+     - Yes
+   * - ``for (auto x : vec)`` (`be careful about the type of the iterator <https://stackoverflow.com/questions/15176104/c11-range-based-loop-get-item-by-value-or-reference-to-const>`__)
+     - 4.6
+     - 3.0
+     - Yes
+   * - ``override``/``final``
+     - 4.7
+     - 3.0
+     - Yes
+   * - ``thread_local``
+     - 4.8
+     - 3.3
+     - **No** (see notes)
+   * - function template default arguments
+     - 4.3
+     - 2.9
+     - Yes
+   * - local structs as template parameters
+     - 4.5
+     - 2.9
+     - Yes
+   * - extended friend declarations
+     - 4.7
+     - 2.9
+     - Yes
+   * - ``0b100`` (C++14)
+     - 4.9
+     - 2.9
+     - Yes
+   * - `Tweaks to some C++ contextual conversions` (C++14)
+     - 4.9
+     - 3.4
+     - Yes
+   * - Return type deduction (C++14)
+     - 4.9
+     - 3.4
+     - Yes (but only in template code when you would have used ``decltype (complex-expression)``)
+   * - Generic lambdas (C++14)
+     - 4.9
+     - 3.4
+     - Yes
+   * - Initialized lambda captures (C++14)
+     - 4.9
+     - 3.4
+     - Yes
+   * - Digit separator (C++14)
+     - 4.9
+     - 3.4
+     - Yes
+   * - Variable templates (C++14)
+     - 5.0
+     - 3.4
+     - Yes
+   * - Relaxed constexpr (C++14)
+     - 5.0
+     - 3.4
+     - Yes
+   * - Aggregate member initialization (C++14)
+     - 5.0
+     - 3.3
+     - Yes
+   * - Clarifying memory allocation (C++14)
+     - 5.0
+     - 3.4
+     - Yes
+   * - [[deprecated]] attribute (C++14)
+     - 4.9
+     - 3.4
+     - **No** (see notes)
+   * - Sized deallocation (C++14)
+     - 5.0
+     - 3.4
+     - **No** (see notes)
+   * - Concepts (Concepts TS)
+     - 6.0
+     - —
+     - **No**
+   * - Inline variables (C++17)
+     - 7.0
+     - 3.9
+     - Yes
+   * - constexpr_if (C++17)
+     - 7.0
+     - 3.9
+     - Yes
+   * - constexpr lambdas (C++17)
+     - —
+     - —
+     - **No**
+   * - Structured bindings (C++17)
+     - 7.0
+     - 4.0
+     - Yes
+   * - Separated declaration and condition in ``if``, ``switch`` (C++17)
+     - 7.0
+     - 3.9
+     - Yes
+   * - `Fold expressions <https://en.cppreference.com/w/cpp/language/fold>`__ (C++17)
+     - 6.0
+     - 3.9
+     - Yes
+   * - [[fallthrough]],  [[maybe_unused]], [[nodiscard]] (C++17)
+     - 7.0
+     - 3.9
+     - Yes
+   * - Aligned allocation/deallocation (C++17)
+     - 7.0
+     - 4.0
+     - **No** (see notes)
+   * - Designated initializers (C++20)
+     - 8.0 (4.7)
+     - 10.0 (3.0)
+     - Yes [*sic*] (see notes)
+   * - #pragma once
+     - 3.4
+     - Yes
+     - **Not** until we `normalize headers <https://groups.google.com/d/msg/mozilla.dev.platform/PgDjWw3xp8k/eqCFlP4Kz1MJ>`__
+   * - `Source code information capture <https://en.cppreference.com/w/cpp/experimental/lib_extensions_2#Source_code_information_capture>`__
+     - 8.0
+     - —
+     - **No**
+
+Sources
+~~~~~~~
+
+* GCC: https://gcc.gnu.org/projects/cxx-status.html
+* Clang: https://clang.llvm.org/cxx_status.html
+
+Notes
+~~~~~
+
+rvalue references
+  Implicit move method generation cannot be used.
+
+Attributes
+  Several common attributes are defined in
+  `mozilla/Attributes.h <https://searchfox.org/mozilla-central/source/mfbt/Attributes.h>`__
+  or nscore.h.
+
+Alignment
+  Some alignment utilities are defined in `mozilla/Alignment.h
+  <https://searchfox.org/mozilla-central/source/mfbt/Alignment.h>`__.
+
+  .. caution::
+    ``MOZ_ALIGNOF`` and ``alignof`` don't have the same semantics. Be careful of what you
+    expect from them.
+
+``[[deprecated]]``
+  If we have deprecated code, we should be removing it rather than marking it as
+  such. Marking things as ``[[deprecated]]`` also means the compiler will warn
+  if you use the deprecated API, which turns into a fatal error in our
+  automation builds, which is not helpful.
+
+Sized deallocation
+  Our compilers all support this (custom flags are required for GCC and Clang),
+  but turning it on breaks some classes' ``operator new`` methods, and `some
+  work <https://bugzilla.mozilla.org/show_bug.cgi?id=1250998>`__ would need to
+  be done to make it an efficiency win with our custom memory allocator.
+
+Aligned allocation/deallocation
+  Our custom memory allocator doesn't have support for these functions.
+
+Thread locals
+  ``thread_local`` is not supported on Android.
+
+Designated initializers
+  Despite their late addition to C++ (and lack of *official* support by
+  compilers until relatively recently), `C++20's designated initializers
+  <https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0329r4.pdf>`__ are
+  merely a subset of `a feature originally introduced in C99
+  <https://gcc.gnu.org/onlinedocs/gcc/Designated-Inits.html>`__ -- and this
+  subset has been accepted without comment in C++ code since at least GCC 4.7
+  and Clang 3.0.
+
+
+C++ and Mozilla standard libraries
+----------------------------------
+
+The Mozilla codebase contains within it several subprojects which follow
+different rules for which libraries can and can't be used it. The rules
+listed here apply to normal platform code, and assume unrestricted
+usability of MFBT or XPCOM APIs.
+
+.. warning::
+
+   The rest of this section is a draft for expository and exploratory
+   purposes. Do not trust the information listed here.
+
+What follows is a list of standard library components provided by
+Mozilla or the C++ standard. If an API is not listed here, then it is
+not permissible to use it in Mozilla code. Deprecated APIs are not
+listed here. In general, prefer Mozilla variants of data structures to
+standard C++ ones, even when permitted to use the latter, since Mozilla
+variants tend to have features not found in the standard library (e.g.,
+memory size tracking) or have more controllable performance
+characteristics.
+
+A list of approved standard library headers is maintained in
+`config/stl-headers.mozbuild <https://searchfox.org/mozilla-central/source/config/stl-headers.mozbuild>`__.
+
+
+Data structures
+~~~~~~~~~~~~~~~
+
+.. list-table::
+   :widths: 25 25 25 25
+   :header-rows: 1
+
+   * - Name
+     - Header
+     - STL equivalent
+     - Notes
+   * - ``nsAutoTArray``
+     - ``nsTArray.h``
+     -
+     - Like ``nsTArray``, but will store a small amount as stack storage
+   * - ``nsAutoTObserverArray``
+     - ``nsTObserverArray.h``
+     -
+     - Like ``nsTObserverArray``, but will store a small amount as stack storage
+   * - ``mozilla::BloomFilter``
+     - ``mozilla/BloomFilter.h``
+     -
+     - Probabilistic set membership (see `Wikipedia <https://en.wikipedia.org/wiki/Bloom_filter#Counting_filters>`__)
+   * - ``nsClassHashtable``
+     - ``nsClassHashtable.h``
+     -
+     - Adaptation of nsTHashtable, see :ref:`XPCOM Hashtable Guide`
+   * - ``nsCOMArray``
+     - ``nsCOMArray.h``
+     -
+     - Like ``nsTArray<nsCOMPtr<T>>``
+   * - ``nsDataHashtable``
+     - ``nsClassHashtable.h``
+     - ``std::unordered_map``
+     - Adaptation of ``nsTHashtable``, see :ref:`XPCOM Hashtable Guide`
+   * - ``nsDeque``
+     - ``nsDeque.h``
+     - ``std::deque<void *>``
+     -
+   * - ``mozilla::EnumSet``
+     - ``mozilla/EnumSet.h``
+     -
+     - Like ``std::set``, but for enum classes.
+   * - ``mozilla::Hash{Map,Set}``
+     - `mozilla/HashTable.h <https://searchfox.org/mozilla-central/source/mfbt/HashTable.h>`__
+     - ``std::unordered_{map,set}``
+     - A general purpose hash map and hash set.
+   * - ``nsInterfaceHashtable``
+     - ``nsInterfaceHashtable.h``
+     - ``std::unordered_map``
+     - Adaptation of ``nsTHashtable``, see :ref:`XPCOM Hashtable Guide`
+   * - ``mozilla::LinkedList``
+     - ``mozilla/LinkedList.h``
+     - ``std::list``
+     - Doubly-linked list
+   * - ``nsRef PtrHashtable``
+     - ``nsRefPtrHashtable.h``
+     - ``std::unordered_map``
+     - Adaptation of ``nsTHashtable``, see :ref:`XPCOM Hashtable Guide`
+   * - ``mozilla::SegmentedVector``
+     - ``mozilla/SegmentedVector.h``
+     - ``std::deque`` w/o O(1) pop_front
+     - Doubly-linked list of vector elements
+   * - ``mozilla::SplayTree``
+     - ``mozilla/SplayTree.h``
+     -
+     - Quick access to recently-accessed elements (see `Wikipedia <https://en.wikipedia.org/wiki/Splay_tree>`__)
+   * - ``nsTArray``
+     - ``nsTArray.h``
+     - ``std::vector``
+     -
+   * - ``nsTHashtable``
+     - ``nsTHashtable.h``
+     - ``std::unordered_{map,set}``
+     - See :ref:`XPCOM Hashtable Guide`,  you probably want a subclass
+   * - ``nsTObserverArray``
+     - ``nsTObserverArray.h``
+     -
+     - Like ``nsTArray``, but iteration is stable even through mutation
+   * - ``nsTPriorityQueue``
+     - ``nsTPriorityQueue.h``
+     - ``std::priority_queue``
+     - Unlike the STL class, not a container adapter
+   * - ``mozilla::Vector``
+     - ``mozilla/Vector.h``
+     - ``std::vector``
+     -
+   * - ``mozilla::Buffer``
+     - ``mozilla/Buffer.h``
+     -
+     - Unlike ``Array``, has a run-time variable length. Unlike ``Vector``, does not have capacity and growth mechanism. Unlike  ``Span``, owns  its buffer.
+
+
+Safety utilities
+~~~~~~~~~~~~~~~~
+
+.. list-table::
+   :widths: 25 25 25 25
+   :header-rows: 1
+
+   * - Name
+     - Header
+     - STL equivalent
+     - Notes
+   * - ``mozilla::Array``
+     - ``mfbt/Array.h``
+     -
+     - safe array index
+   * - ``mozilla::AssertedCast``
+     - ``mfbt/Casting.h``
+     -
+     - casts
+   * - ``mozilla::CheckedInt``
+     - ``mfbt/CheckedInt.h``
+     -
+     - avoids overflow
+   * - ``nsCOMPtr``
+     - ``xpcom/base/nsCOMPtr.h``
+     - ``std::shared_ptr``
+     -
+   * - ``mozilla::EnumeratedArray``
+     - ``mfbt/EnumeratedArray.h``
+     - ``mozilla::Array``
+     -
+   * - ``mozilla::Maybe``
+     - ``mfbt/Maybe.h``
+     - ``std::optional``
+     -
+   * - ``mozilla::RangedPtr``
+     - ``mfbt/RangedPtr.h``
+     -
+     - like ``mozilla::Span`` but with two pointers instead of pointer and length
+   * - ``mozilla::RefPtr``
+     - ``mfbt/RefPtr.h``
+     - ``std::shared_ptr``
+     -
+   * - ``mozilla::Span``
+     - ``mozilla/Span.h``
+     - ``gsl::span``, ``absl::Span``, ``std::string_view``, ``std::u16string_view``
+     - Rust's slice concept for C++ (without borrow checking)
+   * - ``StaticRefPtr``
+     - ``xpcom/base/StaticPtr.h``
+     -
+     - ``nsRefPtr`` w/o static constructor
+   * - ``mozilla::UniquePtr``
+     - ``mfbt/UniquePtr.h``
+     - ``std::unique_ptr``
+     -
+   * - ``mozilla::WeakPtr``
+     - ``mfbt/WeakPtr.h``
+     - ``std::weak_ptr``
+     -
+   * - ``nsWeakPtr``
+     - ``xpcom/base/nsWeakPtr.h``
+     - ``std::weak_ptr``
+     -
+
+
+Strings
+~~~~~~~
+
+See the :doc:`Mozilla internal string guide </xpcom/stringguide>` for
+usage of ``nsAString`` (our copy-on-write replacement for
+``std::u16string``) and ``nsACString`` (our copy-on-write replacement
+for ``std::string``).
+
+Be sure not to introduce further uses of ``std::wstring``, which is not
+portable! (Some uses exist in the IPC code.)
+
+
+Algorithms
+~~~~~~~~~~
+
+.. list-table::
+   :widths: 25 25
+
+   * - ``mozilla::BinarySearch``
+     - ``mfbt/BinarySearch.h``
+   * - ``mozilla::BitwiseCast``
+     - ``mfbt/Casting.h`` (strict aliasing-safe cast)
+   * - ``mozilla/MathAlgorithms.h``
+     - (rotate, ctlz, popcount, gcd, abs, lcm)
+   * - ``mozilla::RollingMean``
+     - ``mfbt/RollingMean.h`` ()
+
+
+Concurrency
+~~~~~~~~~~~
+
+.. list-table::
+   :widths: 25 25 25 25
+   :header-rows: 1
+
+   * - Name
+     - Header
+     - STL/boost equivalent
+     - Notes
+   * - ``mozilla::Atomic``
+     - mfbt/Atomic.h
+     - ``std::atomic``
+     -
+   * - ``mozilla::CondVar``
+     - xpcom/threads/CondVar.h
+     - ``std::condition_variable``
+     -
+   * - ``mozilla::DataMutex``
+     - xpcom/threads/DataMutex.h
+     - ``boost::synchronized_value``
+     -
+   * - ``mozilla::Monitor``
+     - xpcom/threads/Monitor.h
+     -
+     -
+   * - ``mozilla::Mutex``
+     - xpcom/threads/Mutex.h
+     - ``std::mutex``
+     -
+   * - ``mozilla::ReentrantMonitor``
+     - xpcom/threads/ReentrantMonitor.h
+     -
+     -
+   * - ``mozilla::StaticMutex``
+     - xpcom/base/StaticMutex.h
+     - ``std::mutex``
+     - Mutex that can (and in fact, must) be used as a global/static variable.
+
+
+Miscellaneous
+~~~~~~~~~~~~~
+
+.. list-table::
+   :widths: 25 25 25 25
+   :header-rows: 1
+
+   * - Name
+     - Header
+     - STL/boost equivalent
+     - Notes
+   * - ``mozilla::AlignedStorage``
+     - mfbt/Alignment.h
+     - ``std::aligned_storage``
+     -
+   * - ``mozilla::MaybeOneOf``
+     - mfbt/MaybeOneOf.h
+     - ``std::optional<std::variant<T1, T2>>``
+     - ~ ``mozilla::Maybe<union {T1, T2}>``
+   * - ``mozilla::Pair``
+     - mfbt/Pair.h
+     - ``std::tuple<T1, T2>``
+     - minimal space!
+   * - ``mozilla::TimeStamp``
+     - xpcom/ds/TimeStamp.h
+     - ``std::chrono::time_point``
+     -
+   * -
+     - mozilla/PodOperations.h
+     -
+     - C++ versions of ``memset``, ``memcpy``, etc.
+   * -
+     - mozilla/ArrayUtils.h
+     -
+     -
+   * -
+     - mozilla/Compression.h
+     -
+     -
+   * -
+     - mozilla/Endian.h
+     -
+     -
+   * -
+     - mozilla/FloatingPoint.h
+     -
+     -
+   * -
+     - mozilla/HashFunctions.h
+     - ``std::hash``
+     -
+   * -
+     - mozilla/Move.h
+     - ``std::move``, ``std::swap``, ``std::forward``
+     -
+
+
+Mozilla data structures and standard C++ ranges and iterators
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Some Mozilla-defined data structures provide STL-style
+`iterators <https://en.cppreference.com/w/cpp/named_req/Iterator>`__ and
+are usable in `range-based for
+loops <https://en.cppreference.com/w/cpp/language/range-for>`__ as well
+as STL `algorithms <https://en.cppreference.com/w/cpp/algorithm>`__.
+
+Currently, these include:
+
+.. list-table::
+   :widths: 16 16 16 16 16
+   :header-rows: 1
+
+   * - Name
+     - Header
+     - Bug(s)
+     - Iterator category
+     - Notes
+   * - ``nsTArray``
+     - ``xpcom/ds/n sTArray.h``
+     - `1126552 <https://bugzilla.mozilla.org/show_bug.cgi?id=1126552>`__
+     - Random-access
+     - Also reverse-iterable. Also supports remove-erase pattern via RemoveElementsAt method. Also supports back-inserting output iterators via ``MakeBackInserter`` function.
+   * - ``nsBaseHashtable`` and subclasses: ``nsClassHashtable`` ``nsDataHashtable`` ``nsInterfaceHashtable`` ``nsJSThingHashtable`` ``nsRefPtrHashtable``
+     - ``xpcom/ds/nsBaseHashtable.h`` ``xpcom/ds/nsClassHashtable.h`` ``xpcom/ds/nsDataHashtable.h`` ``xpcom/ds/nsInterfaceHashtable.h`` ``xpcom/ds/nsJSThingHashtable.h`` ``xpcom/ds/nsRefPtrHashtable.h``
+     - `1575479 <https://bugzilla.mozilla.org/show_bug.cgi?id=1575479>`__
+     - Forward
+     -
+   * - ``nsCOMArray``
+     - ``xpcom/ds/nsCOMArray.h``
+     - `1342303 <https://bugzilla.mozilla.org/show_bug.cgi?id=1342303>`__
+     - Random-access
+     - Also reverse-iterable.
+   * - ``Array`` ``EnumerationArray`` ``RangedArray``
+     - ``mfbt/Array.h`` ``mfbt/EnumerationArray.h`` ``mfbt/RangedArray.h``
+     - `1216041 <https://bugzilla.mozilla.org/show_bug.cgi?id=1216041>`__
+     - Random-access
+     - Also reverse-iterable.
+   * - ``Buffer``
+     - ``mfbt/Buffer.h``
+     - `1512155 <https://bugzilla.mozilla.org/show_bug.cgi?id=1512155>`__
+     - Random-access
+     - Also reverse-iterable.
+   * - ``DoublyLinkedList``
+     - ``mfbt/DoublyLinkedList.h``
+     - `1277725 <https://bugzilla.mozilla.org/show_bug.cgi?id=1277725>`__
+     - Forward
+     -
+   * - ``EnumeratedRange``
+     - ``mfbt/EnumeratedRange.h``
+     - `1142999 <https://bugzilla.mozilla.org/show_bug.cgi?id=1142999>`__
+     - *Missing*
+     - Also reverse-iterable.
+   * - ``IntegerRange``
+     - ``mfbt/IntegerRange.h``
+     - `1126701 <https://bugzilla.mozilla.org/show_bug.cgi?id=1126701>`__
+     - *Missing*
+     - Also reverse-iterable.
+   * - ``SmallPointerArray``
+     - ``mfbt/SmallPointerArray.h``
+     - `1331718 <https://bugzilla.mozilla.org/show_bug.cgi?id=1331718>`__
+     - Random-access
+     -
+   * - ``Span``
+     - ``mfbt/Span.h``
+     - `1295611 <https://bugzilla.mozilla.org/show_bug.cgi?id=1295611>`__
+     - Random-access
+     - Also reverse-iterable.
+
+Note that if the iterator category is stated as "missing", the type is
+probably only usable in range-based for. This is most likely just an
+omission, which could be easily fixed.
+
+Useful in this context are also the class template ``IteratorRange``
+(which can be used to construct a range from any pair of iterators) and
+function template ``Reversed`` (which can be used to reverse any range),
+both defined in ``mfbt/ReverseIterator.h``
+
+
+Further C++ rules
+-----------------
+
+
+Don't use static constructors
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+(You probably shouldn't be using global variables to begin with. Quite
+apart from the weighty software-engineering arguments against them,
+globals affect startup time! But sometimes we have to do ugly things.)
+
+Non-portable example:
+
+.. code-block:: c++
+
+   FooBarClass static_object(87, 92);
+
+   void
+   bar()
+   {
+     if (static_object.count > 15) {
+        ...
+     }
+   }
+
+Once upon a time, there were compiler bugs that could result in
+constructors not being called for global objects. Those bugs are
+probably long gone by now, but even with the feature working correctly,
+there are so many problems with correctly ordering C++ constructors that
+it's easier to just have an init function:
+
+.. code-block:: c++
+
+   static FooBarClass* static_object;
+
+   FooBarClass*
+   getStaticObject()
+   {
+     if (!static_object)
+       static_object =
+         new FooBarClass(87, 92);
+     return static_object;
+   }
+
+   void
+   bar()
+   {
+     if (getStaticObject()->count > 15) {
+       ...
+     }
+   }
+
+
+Don't use exceptions
+~~~~~~~~~~~~~~~~~~~~
+
+See the introduction to the "C++ language features" section at the start
+of this document.
+
+
+Don't use Run-time Type Information
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+See the introduction to the "C++ language features" section at the start
+of this document.
+
+If you need runtime typing, you can achieve a similar result by adding a
+``classOf()`` virtual member function to the base class of your
+hierarchy and overriding that member function in each subclass. If
+``classOf()`` returns a unique value for each class in the hierarchy,
+you'll be able to do type comparisons at runtime.
+
+
+Don't use the C++ standard library (including iostream and locale)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+See the section "C++ and Mozilla standard libraries".
+
+
+Use C++ lambdas, but with care
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+C++ lambdas are supported across all our compilers now. Rejoice! We
+recommend explicitly listing out the variables that you capture in the
+lambda, both for documentation purposes, and to double-check that you're
+only capturing what you expect to capture.
+
+
+Use namespaces
+~~~~~~~~~~~~~~
+
+Namespaces may be used according to the style guidelines in :ref:`C++ Coding style`.
+
+
+Don't mix varargs and inlines
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+What? Why are you using varargs to begin with?! Stop that at once!
+
+
+Make header files compatible with C and C++
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Non-portable example:
+
+.. code-block:: c++
+
+   /*oldCheader.h*/
+   int existingCfunction(char*);
+   int anotherExistingCfunction(char*);
+
+   /* oldCfile.c */
+   #include "oldCheader.h"
+   ...
+
+   // new file.cpp
+   extern "C" {
+   #include "oldCheader.h"
+   };
+   ...
+
+If you make new header files with exposed C interfaces, make the header
+files work correctly when they are included by both C and C++ files.
+
+(If you need to include a C header in new C++ files, that should just
+work. If not, it's the C header maintainer's fault, so fix the header if
+you can, and if not, whatever hack you come up with will probably be
+fine.)
+
+Portable example:
+
+.. code-block:: c++
+
+   /* oldCheader.h*/
+   PR_BEGIN_EXTERN_C
+   int existingCfunction(char*);
+   int anotherExistingCfunction(char*);
+   PR_END_EXTERN_C
+
+   /* oldCfile.c */
+   #include "oldCheader.h"
+   ...
+
+   // new file.cpp
+   #include "oldCheader.h"
+   ...
+
+There are number of reasons for doing this, other than just good style.
+For one thing, you are making life easier for everyone else, doing the
+work in one common place (the header file) instead of all the C++ files
+that include it. Also, by making the C header safe for C++, you document
+that "hey, this file is now being included in C++". That's a good thing.
+You also avoid a big portability nightmare that is nasty to fix...
+
+
+Use override on subclass virtual member functions
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The ``override`` keyword is supported in C++11 and in all our supported
+compilers, and it catches bugs.
+
+
+Always declare a copy constructor and assignment operator
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Many classes shouldn't be copied or assigned. If you're writing one of
+these, the way to enforce your policy is to declare a deleted copy
+constructor as private and not supply a definition. While you're at it,
+do the same for the assignment operator used for assignment of objects
+of the same class. Example:
+
+.. code-block:: c++
+
+   class Foo {
+     ...
+     private:
+       Foo(const Foo& x) = delete;
+       Foo& operator=(const Foo& x) = delete;
+   };
+
+Any code that implicitly calls the copy constructor will hit a
+compile-time error. That way nothing happens in the dark. When a user's
+code won't compile, they'll see that they were passing by value, when
+they meant to pass by reference (oops).
+
+
+Be careful of overloaded methods with like signatures
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+It's best to avoid overloading methods when the type signature of the
+methods differs only by one "abstract" type (e.g. ``PR_Int32`` or
+``int32``). What you will find as you move that code to different
+platforms, is suddenly on the Foo2000 compiler your overloaded methods
+will have the same type-signature.
+
+
+Type scalar constants to avoid unexpected ambiguities
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Non-portable code:
+
+.. code-block:: c++
+
+   class FooClass {
+     // having such similar signatures
+     // is a bad idea in the first place.
+     void doit(long);
+     void doit(short);
+   };
+
+   void
+   B::foo(FooClass* xyz)
+   {
+     xyz->doit(45);
+   }
+
+Be sure to type your scalar constants, e.g., ``uint32_t(10)`` or
+``10L``. Otherwise, you can produce ambiguous function calls which
+potentially could resolve to multiple methods, particularly if you
+haven't followed (2) above. Not all of the compilers will flag ambiguous
+method calls.
+
+Portable code:
+
+.. code-block:: c++
+
+   class FooClass {
+     // having such similar signatures
+     // is a bad idea in the first place.
+     void doit(long);
+     void doit(short);
+   };
+
+   void
+   B::foo(FooClass* xyz)
+   {
+     xyz->doit(45L);
+   }
+
+
+Use nsCOMPtr in XPCOM code
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+See the ``nsCOMPtr`` `User
+Manual <https://developer.mozilla.org/en-US/docs/Using_nsCOMPtr>`__ for
+usage details.
+
+
+Don't use identifiers that start with an underscore
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+This rule occasionally surprises people who've been hacking C++ for
+decades. But it comes directly from the C++ standard!
+
+According to the C++ Standard, 17.4.3.1.2 Global Names
+[lib.global.names], paragraph 1:
+
+Certain sets of names and function signatures are always reserved to the
+implementation:
+
+-  Each name that contains a double underscore (__) or begins with an
+   underscore followed by an uppercase letter (2.11) is reserved to the
+   implementation for any use.
+-  **Each name that begins with an underscore is reserved to the
+   implementation** for use as a name in the global namespace.
+
+
+Stuff that is good to do for C or C++
+-------------------------------------
+
+
+Avoid conditional #includes when possible
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Don't write an ``#include`` inside an ``#ifdef`` if you could instead
+put it outside. Unconditional includes are better because they make the
+compilation more similar across all platforms and configurations, so
+you're less likely to cause stupid compiler errors on someone else's
+favorite platform that you never use.
+
+Bad code example:
+
+.. code-block:: c++
+
+   #ifdef MOZ_ENABLE_JPEG_FOUR_BILLION
+   #include <stdlib.h>   // <--- don't do this
+   #include "jpeg4e9.h"  // <--- only do this if the header really might not be there
+   #endif
+
+Of course when you're including different system files for different
+machines, you don't have much choice. That's different.
+
+
+Every .cpp source file should have a unique name
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Every object file linked into libxul needs to have a unique name. Avoid
+generic names like nsModule.cpp and instead use nsPlacesModule.cpp.
+
+
+Turn on warnings for your compiler, and then write warning free code
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+What generates a warning on one platform will generate errors on
+another. Turn warnings on. Write warning-free code. It's good for you.
+Treat warnings as errors by adding
+``ac_add_options --enable-warnings-as-errors`` to your mozconfig file.
+
+
+Use the same type for all bitfields in a ``struct`` or ``class``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Some compilers do not pack the bits when different bitfields are given
+different types. For example, the following struct might have a size of
+8 bytes, even though it would fit in 1:
+
+.. code-block:: c++
+
+   struct {
+     char ch: 1;
+     int i: 1;
+   };
+
+
+Don't use an enum type for a bitfield
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The classic example of this is using ``PRBool`` for a boolean bitfield.
+Don't do that. ``PRBool`` is a signed integer type, so the bitfield's
+value when set will be ``-1`` instead of ``+1``, which---I know,
+*crazy*, right? The things C++ hackers used to have to put up with...
+
+You shouldn't be using ``PRBool`` anyway. Use ``bool``. Bitfields of
+type ``bool`` are fine.
+
+Enums are signed on some platforms (in some configurations) and unsigned
+on others and therefore unsuitable for writing portable code when every
+bit counts, even if they happen to work on your system.