Stray start tag “caption”.

A <caption> element only belongs inside a <table>, as its first child, so the validator reports any <caption> start tag it finds anywhere else.

The <caption> element is the title of a table. When the HTML parser meets a <caption> tag outside a table, it drops the tag: the DOM gets no caption element, and whatever was inside it becomes plain content of the surrounding element. The parser drops the closing tag as well, so the same spot usually also gets a "Stray end tag caption" error.

A common source is an image caption written as <caption> around an <img>. WordPress's [caption] shortcode turns into this markup when its square brackets get swapped for angle brackets, for example after pasting content from one editor into another. To caption an image, use a <figure> with a <figcaption> inside it.

If the caption belongs to a table, move it inside the <table> as its first child. A <caption> inside the table but after the rows gets a different error, saying caption isn't allowed as a child of table in that context.

Invalid example

<div class="photo">
  <caption>
    <img src="harbor.jpg" alt="Fishing boats moored in a harbor">
    Fishing boats in the harbor at sunrise.
  </caption>
</div>

<caption>Opening hours</caption>
<table>
  <tr>
    <th>Monday</th>
    <td>9:00 to 17:00</td>
  </tr>
</table>

Valid example

<figure class="photo">
  <img src="harbor.jpg" alt="Fishing boats moored in a harbor">
  <figcaption>Fishing boats in the harbor at sunrise.</figcaption>
</figure>

<table>
  <caption>Opening hours</caption>
  <tr>
    <th>Monday</th>
    <td>9:00 to 17:00</td>
  </tr>
</table>

Learn more:

MDN Reference

MDN Web Docs: <caption>

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MDN Reference

MDN Web Docs: <figcaption>

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WHATWG Spec

HTML Living Standard: Tabular data

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