Mark the defining instance of a term with dfn
The dfn element marks the place where a specialist term gets defined. It wraps the term and not the definition, and the definition still has to appear in the words around it. W3C attaches H54 to one criterion, 3.1.3 Unusual Words, which is Level AAA, and lists it as sufficient only in combination with the general technique of giving definitions inline. So dfn is the marker and the sentence is the substance. Where it earns its keep is jargon-heavy writing, because it gives glossary tooling and search something to hook onto, and because deciding to mark a defining instance forces an author to check that one exists.
How we find it in an audit
No tool can tell which words in your copy are unusual or whether they were ever explained, so this is editorial review from end to end. When a client asks us to look past Level AA we read for undefined jargon and check that the first use genuinely defines the term rather than merely using it in a confident sentence.
How affected users experience it
This one helps comprehension broadly rather than serving any single assistive technology. Readers with cognitive disabilities, readers working in a second language, and anybody meeting a trade vocabulary for the first time all get the same benefit from a term that was actually defined. The discipline is the real payoff, because the markup only goes in where the writing did its job.
Passes vs. fails
Passes
<p>The <dfn>waterfall</dfn> is the order in which proceeds are paid out to investors.</p>Fails
A prospectus that uses the word waterfall four times and never says what it means.
Other ways to satisfy this rule
7 guides on this site are filed under 3.1.3 Unusual Words. W3C lists this one as sufficient for that rule only alongside G112, so the pair is what passes and neither half does on its own.
- G55sufficientLink unusual words to their definitions
- G62sufficientProvide a glossary for specialized terms
- G70sufficientOffer a dictionary search for specialized content
- G101sufficientDefine words used in an unusual or restricted way
- G112sufficientDefine specialized terms inline where they appear
- H40sufficientMark up term-and-definition pairs as description lists
This guide is our interpretation of W3C technique H54: Using the dfn element to identify the defining instance of a word or phrase. W3C publishes its techniques as guidance rather than as the standard, and says so on every one of them. The success criterion is what conformance is measured against, and a technique is one documented way to meet it.