docshtmlcheck fails CI when Markdown under docs/ contains invalid inline HTML that the documentation site's renderer silently drops or mangles. It runs as make lint/docs-html (part of make lint).

• Swallowed angle-bracket placeholders. An unwrapped placeholder such as <region> or <server>__ is parsed as an unknown HTML tag and stripped from the rendered page, so readers see broken text. Wrap placeholders in backticks so they render as inline code (see docs/about/contributing/documentation.md). This also covers CLI --help strings and Swagger annotations, whose text is generated into docs/reference/**.
• Void-element end tags such as </br>. Void elements like <br>, <img>, and <hr> have no end tag.
• Capitalized or unregistered component tags such as <Image> or <Table>. The docs renderer reads a capitalized tag as a component reference and drops it unless the component is registered (only the lowercase <children> directive is). Any name outside the standard HTML5 element set is reported the same way.
• Unclosed container tags, for example a <div class="tabs"> that is never closed and leaks its wrapper over the rest of the page.

Each file is parsed with goldmark and only raw-HTML nodes are inspected, so angle brackets inside fenced code blocks, inline code spans, HTML comments, and <https://…> / <user@host> autolinks are ignored. Each raw-HTML node is tokenized as a whole with golang.org/x/net/html, so a tag whose attributes wrap across lines is not torn in half. A tag whose raw name is capitalized is reported as a component reference; otherwise any name outside the standard HTML5 element set (plus the intentional <children> renderer component, which is still balance-checked) is reported. Inline SVG and MathML are intentionally not in the allowed set (no docs page uses them); add the element to allowedElements in main.go if that changes. A finding on a generated page under docs/reference/** also prints a note pointing at the generator source, since edits to the generated file do not persist.

A few gaps are accepted because no docs page hits them today:

• A placeholder whose name is itself a real HTML element (<input>, <time>) is indistinguishable from intended markup and passes. Such placeholders almost always live in fenced code blocks, which are ignored.
• The interior of a raw-text element (<script>, <style>) is a single opaque token to the HTML tokenizer, so a tag nested inside one is not scanned. An unclosed <script>/<style> is still caught.

$ go run ./scripts/docshtmlcheck        # scans docs/
$ go run ./scripts/docshtmlcheck path/to/file.md path/to/dir

allowedUnknownTags in main.go is a deliberately narrow, per-file escape hatch for placeholders whose source is outside this repository (so they cannot be fixed by a source edit here). It currently holds a temporary entry for docs/reference/cli/agent-firewall.md (<host>/<glob>, generated from the external github.com/coder/boundary CLI help).

The escape hatch is self-clearing: if an allowlisted tag no longer appears in its file (for example once the upstream fix and dependency bump land and the generated page no longer emits the bare placeholders), the linter reports a stale-allowlist-entry and fails until the dead entry is removed, so a suppression can never silently mask a later regression of the same tag.