HTML Guide

The controlslist attribute on the <video> element is experimental, and not yet valid in the current HTML5 standard, so you should use it with caution as it may not be supported by all browsers.

The width and height attributes on <img> elements expect a digit to specify the dimension in pixels. It should not contain units, letters or percent signs.

You can achieve this using CSS instead, for example:

<!-- Invalid syntax, the height attribute expects only digits -->
<img src="photo.jpg" alt="cat" height="auto" />

<!-- Valid syntax using CSS -->
<img src="photo.jpg" alt="cat" style="height: auto"  />

The sizes attribute is used to complement the srcset attribute on an <img> tag for responsive images. When this attribute is present, all image candidates must specify its width.

The attributes width and height of <iframe> elements expect a non-negative integer, so an empty string is not allowed. Either define the correct dimension, or remove this attribute.

The attributes width and height of <img> elements expect a non-negative integer, so an empty string is not allowed. Either define the correct dimension, or remove this attribute.

<video> elements accept a height attribute to specify the width in CSS pixels. This value can only be an integer, it should not contain units or %. If you need to specify a percentage width, you can do that with CSS.

Here’s an example of setting width and height on a video element.

<video controls width="640" height="480">
  <source src="/media/cc0-videos/flower.webm" type="video/webm">
</video>

The sizes attribute is used to complement the srcset attribute on an <img> tag for responsive images. When this attribute is present, all image candidates must specify its width.

The attributes width and height on an iframe expect a valid positive integer without any decimals.

Here’s an example of incorrect code where decimals are being used for dimension attributes:

<iframe src="example.html" height="602.88" width="800.2"></iframe>

Corrected code without decimals:

<iframe src="example.html" height="603" width="800"></iframe>

In the corrected code, the width and height values has been changed to a whole number, which conforms to the standard integer value expected by the W3C validator.

The attributes width and height on an iframe expect a valid positive integer without any decimals.

Here’s an example of incorrect code where decimals are being used for dimension attributes:

<img src="photo.jpg" alt="Dog" height="602.88" width="800.2">

Corrected code without decimals:

<img src="photo.jpg" alt="Dog" height="603" width="800">

In the corrected code, the width and height values has been changed to a whole number, which conforms to the standard integer value expected by the W3C validator.

The <iframe> element, used to embed another document inside the current document, accepts both attributes width and height which must be valid non-negative integers. Percentages are not allowed for these attributes.