How to fix: The allowtransparency attribute on the iframe element is obsolete. Use CSS instead.

Most Likely Cause and Fix

Your HTML5 markup contains an <iframe> element with a allowtransparency attribute in the form:

<iframe allowtransparency="0" … />

This IE-specific attribute, which has never been part of a recognised standard, is no longer present in HTML5.

Remove this attribute or wrap it in IE conditional comments:

<iframe <!--[if IE] allowTransparency="true" --> … />

What This Error Means

There are three aspects to a web-based document:

  • Structure: the underlying arrangement of the document and the elements contained within; handled by HTML

  • Presentation: how elements look and where on the page they're placed; handled by CSS.

  • Behaviour: how the elements respond when being interacted with; handled by JavaScript (in general)

The appearance of and placement text and other page elements is a presentational matter and as such is best implemented at the presentational layer and handled by CSS.

HTML5 follows this approach and removes various presentational attributes previously specified on elements in markup in preference of CSS rules that can achieve the equivalent effect.

This error is letting you know that you have presentational markup in your HTML that is no longer valid and that you should instead remove such attributes and apply relevant CSS rules.

How To Fix It

For iframe elements in HTML5 documents

HTML5 no longer allows the allowtransparency attribute on an <iframe> element:

<iframe allowtransparency="true" … />

This IE-specific attribute, which has never been part of a recognised standard, is not present in HTML5.

The validator error suggests that you use CSS instead, however there is no clear CSS option that applies.

Remove this attribute or wrap it in IE conditional comments:

<iframe <!--[if IE] allowTransparency="true" --> … />

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