HTML Guide
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.
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Related W3C validator issues
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 srcset attribute requires a width descriptor (w) or pixel density descriptor (x) for each image candidate when the sizes attribute is present.
When using the sizes attribute on an img element, each entry in srcset must include either a width descriptor (e.g., 860w) or a pixel density descriptor (e.g., 2x). This tells browsers how to select the most appropriate image source for the current viewport or display density. Omitting the descriptor leads to HTML validation errors and unexpected image selection.
Correct usage with width descriptors:
<img
alt=""
sizes="(min-width:568px) 140px"
srcset="photo.png?w=860&q=90 860w"
src="photo.png?w=860&q=90">Correct usage with pixel density descriptors (if sizes is removed):
<img
alt=""
srcset="photo.png?w=860&q=90 2x"
src="photo.png?w=860&q=90">Key points:
- With sizes, use width descriptors (e.g., 860w).
- Without sizes, you may use pixel density descriptors (e.g., 2x).
- Always use either px or w units in the sizes attribute values; do not use w.
A <source> element inside a <picture> that is followed by another <source> or an <img> with srcset must include a media and/or type attribute.
The <source> element is used inside <picture> for responsive images, allowing different resources to be loaded based on conditions such as viewport width (media) or image format (type). According to the HTML standard, when multiple <source> elements are present (or a following <img> with srcset), each <source> must provide a media and/or type attribute so the browser can choose the appropriate resource based on those hints.
Without media or type, the browser cannot distinguish when to use each source, which can lead to validation errors and unexpected rendering behavior.
Incorrect example (causes the validator error):
<picture>
<source srcset="image1.webp">
<source srcset="image2.jpg">
<img alt="" src="fallback.jpg" srcset="fallback2x.jpg 2x">
</picture>Correct examples (fixing the error):
<picture>
<source srcset="image1.webp" type="image/webp">
<source srcset="image2.jpg" type="image/jpeg">
<img alt="" src="fallback.jpg" srcset="fallback2x.jpg 2x">
</picture>or
<picture>
<source srcset="image-small.jpg" media="(max-width: 600px)">
<source srcset="image-large.jpg" media="(min-width: 601px)">
<img alt="" src="fallback.jpg" srcset="fallback2x.jpg 2x">
</picture>By specifying the media and/or type attributes for each <source>, you satisfy the HTML standard and resolve the W3C validator issue.
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 width attribute on the img element must be a positive integer representing the number of pixels.
The HTML img element’s width and height attributes are expected to specify image dimensions in pixels. According to the HTML Living Standard, these attributes accept only non-negative integers. These integers define the rendered dimension of the image, overriding the actual image size based on its native resolution. The value “auto” is not a valid integer, which leads to the validation error you’ve encountered.
Here is a correct usage example of the img element:
<!DOCTYPE html>
<html lang="en">
<head>
<title>Valid Image Width</title>
</head>
<body>
<img src="example.jpg" alt="Example image" width="200" height="100">
</body>
</html>In the example above, the width is set to 200, and the height is set to 100. Both values are non-negative integers representing pixel dimensions. If you intend to maintain the image’s aspect ratio while adjusting another dimension, you can omit one of the attributes, and modern browsers will automatically adjust the aspect ratio based on the given dimension.
The error message is indicating that the width attribute of the <video> element has an invalid value. According to the HTML specification, the width attribute expects a non-negative integer value, representing the pixel width of the video.
The value "auto" is not valid for the width attribute. Instead, specify a full number that indicates the pixel width of the video. If you want the video to be responsive without specifying a fixed width, you can use CSS to achieve that.
Here are two ways to resolve this:
-
Specify a valid pixel value for width:
<video width="640" height="360" controls> <source src="video.mp4" type="video/mp4"> Your browser does not support the video tag. </video> -
Use CSS for a responsive video player:
Instead of using the width attribute, use CSS to set the width of the video element. This allows the video to adjust its size according to the container or viewport.
<style> .responsive-video { width: 100%; height: auto; } </style> <video class="responsive-video" controls> <source src="video.mp4" type="video/mp4"> Your browser does not support the video tag. </video>
In the second example, the video will scale based on its containing element, maintaining its aspect ratio due to the height: auto; CSS rule. This approach offers more flexibility for responsive design.
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.
All values in the srcset attribute must include a width descriptor (such as 300w) when the sizes attribute is present.
The srcset attribute is used to provide multiple image sources for responsive images. Each image candidate string in srcset must specify the image’s width (e.g., 600w) or pixel density (e.g., 2x). When you use a sizes attribute, all srcset candidates must use width descriptors (w).
Example of incorrect usage:
<img
src="/img/pic1.jpg"
srcset="/img/pic1.jpg"
sizes="(max-width: 600px) 100vw, 600px"
alt=""
>This example is invalid because the srcset value does not include a width descriptor.
Corrected usage with width descriptors:
<img
src="/img/pic1.jpg"
srcset="/img/pic1.jpg 600w"
sizes="(max-width: 600px) 100vw, 600px"
alt=""
>If you have multiple image sizes, include each with its corresponding width:
<img
src="/img/pic1.jpg"
srcset="
/img/pic1_small.jpg 300w,
/img/pic1.jpg 600w
"
sizes="(max-width: 600px) 100vw, 600px"
alt=""
>Always match each image URL in srcset with a width (w) or pixel density (x) descriptor if appropriate for your layout.
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
">