The first occurrence of “dt” name “X” was here.

This message points to the first <dt> in a <dl> whose name is repeated later in the same list.

It always comes with a duplicate dt name warning on the later <dt>. Together, the two messages show where each copy of the term is, so you can compare them and decide which one to keep. The HTML specification says a single <dl> should not have more than one <dt> for each name.

The validator compares the text content of each <dt>. It ignores markup and leading or trailing whitespace, so <dt><b>Size</b></dt> and <dt>Size</dt> have the same name. The comparison is case-sensitive, so Size and size don't clash.

The first <dt> is usually fine on its own; the fix goes where the name repeats. If both entries describe the same term, keep one <dt> and give each value its own <dd>. If they describe different things, give each <dt> a distinct name. Once every name in the list is unique, both messages go away.

Invalid example

<dl>
  <dt>Size</dt>
  <dd>Small</dd>
  <dt>Color</dt>
  <dd>Red</dd>
  <dt>Size</dt>
  <dd>Medium</dd>
</dl>

Valid example

<dl>
  <dt>Size</dt>
  <dd>Small</dd>
  <dd>Medium</dd>
  <dt>Color</dt>
  <dd>Red</dd>
</dl>

Learn more:

MDN Reference

MDN Web Docs: The Description List element

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MDN Reference

MDN Web Docs: The Description Term element

→

WHATWG Spec

HTML Standard: Grouping content

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