HTML Guides for revert

The revert keyword is one of the CSS-wide keywords defined in the CSS Cascading and Inheritance specification (along with initial, inherit, and unset). These keywords are valid values for every CSS property. When applied, revert rolls back the cascade to the value the property would have had if no author-level styles were applied — effectively reverting to the browser’s default stylesheet (or the user stylesheet, if one exists).

Older versions of the Nu HTML Checker (the engine behind the W3C HTML Validator) did not fully recognize revert as a valid CSS-wide keyword for all properties, which caused this false positive. This has been fixed in newer versions of the validator. If you’re using a local or outdated instance of the validator, updating to the latest version should resolve the issue.

Since this is a validator bug rather than a code issue, you have a few options: ignore the warning, update your validator, or use an alternative approach if you need a clean validation report. For example, you could use unset instead, which behaves similarly in many cases (though not identically — unset reverts to the inherited or initial value, while revert reverts to the user-agent default).

Examples

Code that triggers the false positive

This inline style uses revert on font-size, which is perfectly valid CSS but may trigger the validator warning:

<p style="font-size: revert;">This paragraph uses the browser's default font size.</p>

Workaround using unset

If you need to pass validation on an older validator instance, unset can serve as a partial substitute. Note that unset behaves differently from revert — it resets to the inherited value (for inherited properties like font-size) rather than the user-agent default:

<p style="font-size: unset;">This paragraph inherits the font size from its parent.</p>

Using revert in a stylesheet

The revert keyword is especially useful when you want to undo author styles for specific elements. This is valid CSS and should not produce errors in up-to-date validators:

<style>
  p {
    font-size: 2rem;
  }
  .default-size {
    font-size: revert;
  }
</style>
<p>This paragraph has a 2rem font size.</p>
<p class="default-size">This paragraph reverts to the browser default font size.</p>

All CSS-wide keywords

For reference, all of these CSS-wide keywords are valid for font-size and every other CSS property:

<div style="font-size: initial;">Uses the property's initial value</div>
<div style="font-size: inherit;">Inherits from the parent element</div>
<div style="font-size: unset;">Resets to inherited or initial value</div>
<div style="font-size: revert;">Reverts to the user-agent default</div>
<div style="font-size: revert-layer;">Reverts to the previous cascade layer</div>