Start tag “form” seen in “table”.

A <form> can't sit directly inside a <table>, <thead>, <tbody>, <tfoot> or <tr>: wrap the whole table in the form, or put the form inside a single <td> or <th> cell.

Table elements only accept specific children. A <table> holds <caption>, <colgroup>, <thead>, <tbody>, <tfoot> and <tr>, and a <tr> holds <td> and <th>. A <form> wrapped around one or more rows breaks that structure.

The browser's parser handles this case in a special way. It creates the form as an empty element at the spot where the tag appears, closes it right away, and drops the matching </form> end tag, which the validator reports as a separate Stray end tag “form” error. Browsers still link the fields in the following rows to that empty form, so submitting often appears to work. But the fields are not inside the form in the DOM: a selector like form input won't find them, and if a script moves or re-renders those rows, the fields lose their form.

When each row needs its own form, such as an "Update" button per cart item, keep the <form> elements outside the table and link each field and button to its form with the form attribute, which takes the form's id.

Invalid example

<table>
  <form action="/cart" method="post">
    <tr>
      <td><label for="qty">Quantity</label></td>
      <td><input type="number" id="qty" name="qty"></td>
      <td><button type="submit">Update</button></td>
    </tr>
  </form>
</table>

Valid example

<form action="/cart" method="post">
  <table>
    <tr>
      <td><label for="qty">Quantity</label></td>
      <td><input type="number" id="qty" name="qty"></td>
      <td><button type="submit">Update</button></td>
    </tr>
  </table>
</form>

Valid example with one form per row

<form id="item-1" action="/cart/1" method="post"></form>
<form id="item-2" action="/cart/2" method="post"></form>
<table>
  <tr>
    <td><input type="number" name="qty" form="item-1" aria-label="Quantity for item 1"></td>
    <td><button type="submit" form="item-1">Update</button></td>
  </tr>
  <tr>
    <td><input type="number" name="qty" form="item-2" aria-label="Quantity for item 2"></td>
    <td><button type="submit" form="item-2">Update</button></td>
  </tr>
</table>

Learn more:

MDN Reference

MDN Web Docs: The form element

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MDN Reference

MDN Web Docs: The table element

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WHATWG Spec

HTML Living Standard: Tabular data

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