HTML Guide

Table rows on the same <table> element must have the same number of columns, which comes determined by the first tr row.

For example, this table is wrong as the first row defines 2 columns, while the second row tries to use 4 columns:

<table>
    <tr>
      <td>Liza</td>
    </tr>

    <tr>
      <td>Jimmy</td>
      <td>14</td>
    </tr>
</table>

A <table> contains a <tr> row that has less <td> columns than the column count established by the first row. Check the table to ensure all rows have the same number of columns.

For example, in the following table, the first <tr> row defines that it’s 2 columns wide, but the second <tr> row tries to use only 1 column:

<table>
<!-- This first row sets the table as 2 columns wide -->

  <tr>
    <td>First</td>
    <td>Second</td>
  </tr>

<!-- This second row has only 1 column -->

  <tr>
    <td>Wrong</td>
  </tr>
</table>

The <table> element does not accept a height attribute. Use CSS instead.

A <div> tag has been found as a direct child of an <ul> tag, and this is not allowed. For example, <ul><div><li>item</li></div></ul> is not valid, but <ul><li><div>item</div></li></ul> is valid as the direct child of <ul> is <li>.

The element X is not allowed as a child element of Y. For example, a <ul> element cannot have a <div> child element.

An </a> end tag has been found to violate nesting rules. <a> tags can’t include other <a> tags inside. Most probable cause is an unclosed <a> tag, like in this example:

<a href="one.html">Page 1
<a href="two.html">Page 2</a>

An end tag </b> has been found in an incorrect place within the document, violating nesting rules. A common case is closing it before closing other nested tags, for example:

<!-- This line is incorrect as the <b> tag was closed before the nested <a> tag -->

<b><a href="#">link</b></a>

<!-- This line is OK as every end tag respects the nesting rules -->

<b><a href="#">link</a></b>

An end tag </code> has been found violating nesting rules. Check other errors in the same document related to the <code> element, and fix the unallowed nested elements.

An end tag </em> has been found in an incorrect place within the document, violating nesting rules. A common case is closing it before closing other nested tags, for example:

<!-- This line is incorrect as the <em> tag was closed before the nested <a> tag -->

<em><a href="#">link</em></a>

<!-- This line is OK as every end tag respects the nesting rules -->

<em><a href="#">link</a></em>

End tag “i” violates nesting rules because the closing </i> does not match the most recently opened still-unclosed inline element, causing mis-nested tags.

HTML requires elements to be properly nested in a stack-like order: last opened, first closed. Inline elements like the i element (for idiomatic text), em, strong, a, and span must not overlap. When you open i and then open b, you must close b before closing i. Mis-nesting often happens around links or emphasis tags, e.g., i inside a but closed outside. The i element is purely presentational/idiomatic; consider em for emphasis or CSS with font-style: italic;, but whichever you use, keep the nesting consistent.

Common patterns that trigger the error:

• Overlapping inline elements: i … b … </i> … </b>
• Closing order mismatch across a, span, em, strong, i
• Copy/paste around icons or screen-reader text

Fix by reordering the tags so they close in reverse order of opening, or by merging styles into one element to avoid overlaps.

HTML Examples

Incorrect (reproduces the validator error)

<p>
  <a href="/about"><i>About <strong>Us</i></strong></a>
</p>

Correct (properly nested)

<p>
  <a href="/about"><i>About <strong>Us</strong></i></a>
</p>