[chrisjsewell]

What and why

Two more HTML-injection points in the #143 family, both reachable with html=False:

• colon_fence (default renderer): the block name was interpolated into class="block-..." unescaped,
  after entities in the info string had been unescaped.
  :::a"onmouseover="alert(1) rendered class="block-a"onmouseover="alert(1)",
  and :::a"><xss> rendered a raw <xss> element.
  The name is now passed through escapeHtml.
• tasklists with label=True, label_after=True: the label is built from the item's source text
  and emitted as an html_inline token, so - [ ] <img src=x onerror=alert(1)> rendered a raw <img>.
  The label text is now passed through escapeHtml.

How it was tested

• New regression tests, both with html=False and exact expected output:
  tests/test_colon_fence.py::test_block_name_is_escaped (raw-quote and entity-encoded payloads, plus a benign &)
  and tests/test_tasklists.py::test_label_after_is_escaped (raw HTML, &, plain text),
  with test_label_after_entity_is_shown_literally pinning how a source entity appears in the label
  (it asserts the label only, as earlier inline children of a label_after item are also rendered outside it,
  an existing quirk not changed here).
• Every case except the plain-text one fails on master and passes with its fix;
  reverting either source file at HEAD makes its tests fail again.
• pytest -q: 518 passed (511 on master plus the 7 new cases), markdown-it-py 4.2.0, Python 3.12.
• pre-commit run --all-files (ruff, ruff-format, mypy): clean.
• No fixture file changed; all existing colon_fence and tasklists fixtures render byte-identical.

Part of #143

This does not close #143. What remains of it is in other open PRs:
the texmath, dollarmath and myst_block escaping and the attrs default (#150),
the ReDoS fixes (#148),
and a test that guards every plugin renderer against XSS (#151).

Notes for the reviewer

• Output changes for benign input:
  a colon_fence block name containing & now emits & (class="block-a&b" instead of class="block-a&b"),
  which is the same attribute value;
  and in tasklists label_after mode, label text containing &, <, > or "
  is now escaped (a & b becomes a & b, which is correct HTML).
  The label is the item's raw source text (it already showed *emphasis* literally),
  so a source entity such as & now displays as the literal text &.
  This applies with html=True as well; on master, inline HTML in a label_after item already produced broken markup.
  No existing fixture uses label_after.
• Adding these two plugins to tests/test_xss.py (which arrives with ✅ TEST(#143): add categorical XSS guard across plugin renderers #151) is left for after those PRs land.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.30%. Comparing base (d11bdaf) to head (3ef7b13).
⚠️ Report is 19 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #165      +/-   ##
==========================================
+ Coverage   92.80%   93.30%   +0.49%     
==========================================
  Files          31       40       +9     
  Lines        1835     2285     +450     
==========================================
+ Hits         1703     2132     +429     
- Misses        132      153      +21     

Flag	Coverage Δ
pytests	93.30% <100.00%> (+0.49%)	⬆️

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