fix: reject invalid characters in HTML tag names - #4083
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styfle
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## [18.0.12](v18.0.11...v18.0.12) (2026-09-07) ### Bug Fixes * allow a tab before the closing sequence of an ATX heading ([#4084](#4084)) ([4417582](4417582)) * allow one more level of nested brackets in a link label ([#4064](#4064)) ([37b28d8](37b28d8)) * do not add a newline to an empty code block ([#4073](#4073)) ([23b1706](23b1706)) * escape character references in autolink destinations ([#4053](#4053)) ([8f432f0](8f432f0)) * reject GFM email autolink when the domain ends in _ or - ([#4063](#4063)) ([df57534](df57534)) * reject invalid characters in HTML tag names ([#4083](#4083)) ([300bb1d](300bb1d)) * remove up to the fence indentation from each content line ([#4074](#4074)) ([0244f08](0244f08))
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Per CommonMark (Raw HTML, and HTML blocks type 7) and GFM, an HTML tag name is
[A-Za-z][A-Za-z0-9-]*. The block and inline tag regexes used[\w-](and[\w:-]for the inline closing tag), so_— and:on closing tags — were accepted. As a result sequences like<bar_baz>,<em_>…</em_>and</a:b>were emitted as raw HTML instead of being escaped as text.This also removes an inconsistency where the inline opening-tag rule rejected
:but the closing-tag rule accepted it.Example:
before:
<p>foo <bar_baz> qux</p>— after:<p>foo <bar_baz> qux</p>(matching commonmark.js).Attribute names still accept
_/:— only tag names are affected. Added atest/specs/newcase covering inline open/close and block tags with_/:. Full spec and unit suites pass.