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HTML lang Attribute Checker

Checks that the html lang and xml:lang attributes match in XHTML and polyglot pages

Source Download skill
Repository
thedaviddias/Front-End-Checklist
Stars
74,632
Licence
MIT
Files
2 files

What it does

When installed, the agent reviews web page markup for a specific accessibility issue: the page's language declared on the <html> element. It checks that lang and xml:lang (used in XHTML or pages meant to work as both HTML and XML) agree, that the language code is a valid BCP 47 tag, and suggests removing xml:lang from ordinary HTML5 pages where it is unnecessary. The goal is to help screen readers read text in the correct language.

Great for

  • Front-end developers auditing XHTML or EPUB-style content for accessibility
  • Teams checking that pages declare their language correctly for screen reader users

Skip it if

  • Modern HTML5-only sites, where only lang is needed and this skill adds little value
  • Pages whose language is unknown or mixed without the author confirming the primary language, since the skill cannot determine that reliably on its own

Try asking

  • “Audit this XHTML file and check that lang and xml:lang on the html element match”
  • “Review my polyglot HTML page for language attribute problems”
  • “Should I remove xml:lang from this HTML5 document?”
Step-by-step guide: how to use HTML lang Attribute CheckerInstall, first run, examples, tips and FAQs

Install in 3 steps

  1. 1

    Download the skill

    The ZIP contains the 2 original files, the MIT licence and an attribution note.

    html-xml-lang-mismatch.zip
  2. 2

    Add it to your agent

    Claude Code (all projects)

    mkdir -p ~/.claude/skills && unzip html-xml-lang-mismatch.zip -d ~/.claude/skills/

    Claude Code / Agent SDK (this project only)

    mkdir -p .claude/skills && unzip html-xml-lang-mismatch.zip -d .claude/skills/

    Claude apps (web/desktop): Settings → Capabilities → Skills → Upload skill, and choose the ZIP.

  3. 3

    Ask for the task

    No special command needed — the agent reads the skill's description and loads it when your request matches. Try one of the prompts above.

Get better results

  • Give the agent the full <html> opening tag or the whole document so it can see both attributes and the content language.
  • Tell the agent whether the page is served as text/html or application/xhtml+xml, since that decides whether xml:lang is needed.
  • Ask it to confirm the primary language of the content if the page mixes languages.

What's inside

  • references/rule.md
  • SKILL.md

Redistributed unmodified from thedaviddias/Front-End-Checklist under the MIT licence. The assessment was drafted with AI from the skill's files on 2026-10-29. Skills can instruct your agent to run code — read SKILL.md and any scripts before installing.