What is a BCP 47 language tag?

BCP 47, defined in RFC 5646, is the standard syntax for language tags such as en, en-GB or zh-Hant-TW. A tag is a primary language subtag optionally followed by script, region, variant and extension subtags, each with its own length and character rules.

Why does the page need a lang attribute?

WCAG 3.1.1 requires the default human language of the page to be programmatically set on the html element. Screen readers use it to choose the correct voice and pronunciation rules, so 'lang equals fr' is read in a French voice rather than mispronounced by an English one.

When do I need inline lang attributes?

WCAG 3.1.2 requires a lang attribute on any passage or phrase in a language different from the page default, such as a French quotation inside an English article. Wrap it in a span with the right lang so assistive technology switches pronunciation for that part only.

What is the difference between well-formed and a known language?

A tag can be syntactically well-formed yet still wrong. For example en-XY parses correctly but the region may not exist. The checker flags primary language subtags that are two letters but not a recognised ISO 639-1 code, since those are usually typos like 'english' instead of 'en'.

Is my HTML uploaded anywhere?

No. Parsing and validation run entirely in your browser using the built-in HTML parser. Nothing you paste is sent to a server or stored.

lang Attribute Checker

A missing or wrong lang attribute is one of the most common and most invisible accessibility defects. The page looks fine, but a screen reader announces French content in an English voice, garbling pronunciation. This checker validates the root document language and every inline language part against the BCP 47 grammar so you can fix both WCAG language criteria at once.

How it works

The tool parses your HTML in the browser, then runs two checks.

Root language (WCAG 3.1.1). It reads the lang attribute on the html element. If absent, that is an immediate failure. If present, the value is validated against BCP 47.

Inline parts (WCAG 3.1.2). It finds every other element carrying a lang attribute and validates each one, reporting a short text preview so you can see which phrase it applies to.

BCP 47 validation

Each tag is split on hyphens and matched against the RFC 5646 subtag grammar:

primary language — 2 to 8 letters (ISO 639). Two-letter codes are additionally checked against the ISO 639-1 list to catch typos.
script — exactly 4 letters (ISO 15924), for example Hant.
region — 2 letters (ISO 3166-1) or 3 digits (UN M.49), for example GB or 419.
variant — 5 to 8 alphanumerics, or 4 characters beginning with a digit.
extension / private use — a single-character singleton followed by more subtags.

A tag passes only when the whole string fits this grammar with no leftover subtags.

Examples

en-GB     valid: English, United Kingdom
zh-Hant   valid: Chinese, Traditional script
fr        valid: French
english   invalid: primary subtag must be a 2 to 8 letter code, not a word
en_US     invalid: BCP 47 uses hyphens, not underscores

Tips

Set lang on html once for the whole page; do not repeat it on every element.
Only add inline lang when the language genuinely changes. A single borrowed word that has entered the main language usually does not need it.
Prefer the shortest valid tag. Use en unless a regional distinction matters, then en-GB or en-US.
All validation is local. Nothing you paste leaves your browser.