HTML Checking for Large Sites

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 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 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.

The srcset property on img elements, when used, requires at least one value. This property is a string which identifies one or more image candidate strings, separated using commas (,) each specifying image resources to use under given circumstances.

Each image candidate string contains an image URL and an optional width or pixel density descriptor that indicates the conditions under which that candidate should be used instead of the image specified by the src property.

Example:

<img
    src="/img/cat-200px.png"
    alt="Cat"
    srcset="
      /img/cat-200px.png 1x,
      /img/cat-400px.png 2x
    ">

The srcset property on source elements, when used, requires at least one value.

The <source> HTML element specifies multiple media resources for the <picture>, the <audio> element, or the <video> element. It is a void element, meaning that it has no content and does not have a closing tag. It is commonly used to offer the same media content in multiple file formats in order to provide compatibility with a broad range of browsers given their differing support for image file formats and media file formats.

The srcset attribute is required if the source element’s parent is a <picture> element, but not allowed if the source element’s parent is an <audio> or <video> element.

The attributes width and height on <img> elements define the dimensions of the image in CSS pixels, and expect a non-negative integer.

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.

<img> elements accept a width attribute to specify the size in 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:

<img src="photo.jpg" alt="red car" style="width:100%;">

<video> elements accept a width 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>