Adding upstream version 2.1.0.upstream/2.1.0upstream

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

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+(md/security)=
+
+# Security
+
+Many people don't understand that markdown format does not care much about security.
+In many cases you have to pass output to sanitizers.
+`markdown-it` provides 2 possible strategies to produce safe output:
+
+1. Don't enable HTML. Extend markup features with [plugins](md/plugins).
+   We think it's the best choice and use it by default.
+   - That's ok for 99% of user needs.
+   - Output will be safe without sanitizer.
+2. Enable HTML and use external sanitizer package(s).
+
+Also by default `markdown-it` prohibits some kind of links, which could be used
+for XSS:
+
+- `javascript:`, `vbscript:`
+- `file:`
+- `data:`, except some images (gif/png/jpeg/webp).
+
+So, by default `markdown-it` should be safe. We care about it.
+
+If you find a security problem - contact us via tracker or email.
+Such reports are fixed with top priority.
+
+## Plugins
+
+Usually, plugins operate with tokenized content, and that's enough to provide safe output.
+
+But there is one non-evident case you should know - don't allow plugins to generate arbitrary element `id` and `name`.
+If those depend on user input - always add prefixes to avoid DOM clobbering.
+See [discussion](https://github.com/markdown-it/markdown-it/issues/28) for details.
+
+So, if you decide to use plugins that add extended class syntax or autogenerating header anchors - be careful.
+
+(md/performance)=
+
+# Performance
+
+You can view our continuous integration benchmarking analysis at: <https://executablebooks.github.io/markdown-it-py/dev/bench/>,
+or you can run it for yourself within the repository:
+
+```console
+$ tox -e py38-bench-packages -- --benchmark-columns mean,stddev
+
+Name (time in ms)             Mean             StdDev
+---------------------------------------------------------------
+test_mistune               70.3272 (1.0)       0.7978 (1.0)
+test_mistletoe            116.0919 (1.65)      6.2870 (7.88)
+test_markdown_it_py       152.9022 (2.17)      4.2988 (5.39)
+test_commonmark_py        326.9506 (4.65)     15.8084 (19.81)
+test_pymarkdown           368.2712 (5.24)      7.5906 (9.51)
+test_pymarkdown_extra     640.4913 (9.11)     15.1769 (19.02)
+test_panflute             678.3547 (9.65)      9.4622 (11.86)
+---------------------------------------------------------------
+```
+
+As you can see, `markdown-it-py` doesn't pay with speed for it's flexibility.
+
+```{note}
+`mistune` is not CommonMark compliant, which is what allows for its
+faster parsing, at the expense of issues, for example, with nested inline parsing.
+See [mistletoes's explanation](https://github.com/miyuchina/mistletoe/blob/master/performance.md)
+for further details.
+```