HTML Guide

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.

Square brackets in an img src query string must be percent-encoded to be valid.

The src attribute on img must be a valid URL. In URL query strings, characters like [ and ] are not allowed unescaped per URL syntax. When present (often from frameworks adding array-like params), they must be percent-encoded as [ -> %5B and ] -> %5D. Alternatively, remove brackets from the query or use a server-side/route format that avoids them.

HTML examples

Example causing the validator error

<img src="/images/photo.jpg?size[width]=300&size[height]=200" alt="Sample">

Fixed example using percent-encoding

<img src="/images/photo.jpg?size%5Bwidth%5D=300&size%5Bheight%5D=200" alt="Sample">

Fixed example by avoiding brackets in params

<img src="/images/photo.jpg?size_width=300&size_height=200" alt="Sample">

The src attribute on an <img> element contains an invalid character, that should be properly encoded as a URI percent-encoded character.

srcset contains candidates without a width descriptor while sizes is present, so each candidate must use a width (w) descriptor.

When an img has sizes, every srcset candidate must include a width descriptor like 320w, not a pixel density descriptor like 1x. Mixed descriptors are not allowed in the same srcset. Use either:

• Width descriptors with sizes (e.g., 320w, 640w, 1024w)
• Density descriptors without sizes (e.g., 1x, 2x)

The browser uses sizes to map CSS layout width to the best w candidate. Without sizes, density (x) can be used, but not together with sizes.

HTML examples

Reproduce the issue (invalid: sizes + x descriptors)

<img
  src="photo-640.jpg"
  srcset="photo-640.jpg 1x, photo-1280.jpg 2x"
  sizes="(max-width: 600px) 100vw, 600px"
  alt="Sample photo">

Fix using width descriptors with sizes (valid)

<img
  src="photo-640.jpg"
  srcset="photo-320.jpg 320w, photo-640.jpg 640w, photo-1280.jpg 1280w"
  sizes="(max-width: 600px) 100vw, 600px"
  alt="Sample photo">

Alternative fix: remove sizes and use density descriptors (valid)

<img
  src="photo-640.jpg"
  srcset="photo-640.jpg 1x, photo-1280.jpg 2x"
  alt="Sample photo">

The src attribute on an element <img> contains a character which is not allowed unless properly encoded.

Special characters needing encoding are: :, /, ?, #, [, ], @, !, $, &, ', (, ), *, +, ,, ;, =, as well as % itself.

For example, this image tag is incorrect because the src attribute contains an the unallowed characters [ and ]:

<img src="image[00].svg" alt="logo">

Instead, this is the properly percent-encoded src attribute, where [ has been replaced with %5B and ] with %5D.

<img src="image%5B00%5D.svg" alt="logo">

Space characters are not allowed in src attributes. Instead, they should be converted to %20. In this example, the first line is invalid and the second is valid:

<img src="https://example.com/?s=some term" alt="description" />
<img src="https://example.com/?s=some%20term" alt="description" />

The src attribute on an <img> tag is not allowed to contain space characters. You should replace them with “%20“.

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
    ">

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&amp;q=90 860w"
  src="photo.png?w=860&amp;q=90">

Correct usage with pixel density descriptors (if sizes is removed):

<img
  alt=""
  srcset="photo.png?w=860&amp;q=90 2x"
  src="photo.png?w=860&amp;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.