Bad value X for attribute “sizes” on element “source”: Bad media condition: Parse Error at Y

The sizes attribute tells the browser how wide an image will be displayed at different viewport sizes, so it can choose the best candidate from srcset. It only applies when srcset uses width descriptors (e.g., 480w, 800w). The value is a comma-separated list where:

1. Each entry (except the last) is a media condition in parentheses followed by a CSS length — for example, (min-width: 800px) 50vw.
2. The final entry is a fallback length with no media condition — for example, 100vw.

The browser evaluates the media conditions from left to right and uses the length from the first one that matches. If none match, it uses the fallback.

The media condition syntax mirrors CSS media queries but without the @media keyword. You can use logical operators like and, or, and not, and features like min-width, max-width, orientation, etc.

Why This Error Occurs

The validator parses the sizes value according to the HTML living standard and CSS Media Queries specification. A parse error is triggered when the value doesn’t conform to the expected grammar. Common causes include:

• Missing parentheses around the media condition: min-width: 600px 50vw instead of (min-width: 600px) 50vw.
• Missing length after a media condition: (min-width: 800px) with no length following it.
• Invalid or missing units on the length: 50 instead of 50vw, 50px, or 50rem.
• Trailing commas or extra separators: (min-width: 600px) 50vw, , 100vw.
• Invalid media feature syntax: typos like (minwidth: 600px) or using unsupported tokens.
• Using sizes with pixel-density descriptors: when srcset uses 1x, 2x instead of width descriptors, the sizes attribute is meaningless and can confuse validators.

Why It Matters

• Browser image selection: Browsers rely on a correctly parsed sizes value to pick the optimal image from srcset. A malformed value causes the browser to fall back to a default size (typically 100vw), which can result in downloading unnecessarily large images on small screens or blurry images on large screens.
• Standards compliance: Invalid sizes values violate the HTML specification and may behave unpredictably across different browsers.
• Performance: Correct sizes values are essential for responsive image optimization. Without them, you lose the bandwidth savings that srcset with width descriptors is designed to provide.

How to Fix It

1. Wrap every media condition in parentheses: (min-width: 600px), not min-width: 600px.
2. Always include a valid CSS length after each condition: (min-width: 600px) 50vw, not just (min-width: 600px).
3. End with a fallback length that has no condition: 100vw.
4. Use valid CSS length units: vw, px, em, rem, or calc() expressions.
5. Remove trailing or duplicate commas.
6. Omit sizes entirely if your srcset uses pixel-density descriptors (1x, 2x).

Examples

Incorrect: missing parentheses around the media condition

<img
  src="photo-800.jpg"
  srcset="photo-400.jpg 400w, photo-800.jpg 800w"
  sizes="min-width: 600px 50vw, 100vw"
  alt="A landscape photo">

Correct: parentheses added

<img
  src="photo-800.jpg"
  srcset="photo-400.jpg 400w, photo-800.jpg 800w"
  sizes="(min-width: 600px) 50vw, 100vw"
  alt="A landscape photo">

Incorrect: missing length after the media condition

<img
  src="banner-800.jpg"
  srcset="banner-400.jpg 400w, banner-800.jpg 800w"
  sizes="(min-width: 700px), 100vw"
  alt="A promotional banner">

The first entry (min-width: 700px) has no length value — the comma makes it look like a separate entry, but it’s incomplete.

Correct: length added after the condition

<img
  src="banner-800.jpg"
  srcset="banner-400.jpg 400w, banner-800.jpg 800w"
  sizes="(min-width: 700px) 60vw, 100vw"
  alt="A promotional banner">

Incorrect: using sizes with pixel-density descriptors

<img
  src="avatar.jpg"
  srcset="avatar@1x.jpg 1x, avatar@2x.jpg 2x"
  sizes="(min-width: 600px) 80px, 40px"
  alt="User avatar">

The sizes attribute is only meaningful with width descriptors (w). When srcset uses density descriptors (1x, 2x), omit sizes.

Correct: sizes removed for density descriptors

<img
  src="avatar.jpg"
  srcset="avatar@1x.jpg 1x, avatar@2x.jpg 2x"
  alt="User avatar">

Incorrect: trailing comma

<img
  src="hero-800.jpg"
  srcset="hero-400.jpg 400w, hero-800.jpg 800w"
  sizes="(min-width: 800px) 800px, 100vw,"
  alt="Hero image">

Correct: trailing comma removed

<img
  src="hero-800.jpg"
  srcset="hero-400.jpg 400w, hero-800.jpg 800w"
  sizes="(min-width: 800px) 800px, 100vw"
  alt="Hero image">

Using sizes on a <source> inside <picture>

<picture>
  <source
    type="image/webp"
    srcset="hero-480.webp 480w, hero-800.webp 800w, hero-1200.webp 1200w"
    sizes="(min-width: 1200px) 1200px, (min-width: 800px) 800px, 100vw">
  <img
    src="hero-800.jpg"
    srcset="hero-480.jpg 480w, hero-800.jpg 800w, hero-1200.jpg 1200w"
    sizes="(min-width: 1200px) 1200px, (min-width: 800px) 800px, 100vw"
    alt="Hero image">
</picture>

Using compound media conditions

You can combine conditions with and:

<img
  src="photo-800.jpg"
  srcset="photo-400.jpg 400w, photo-800.jpg 800w, photo-1200.jpg 1200w"
  sizes="(min-width: 900px) and (orientation: landscape) 50vw, 100vw"
  alt="A sample photo">

Full valid document

<!doctype html>
<html lang="en">
  <head>
    <meta charset="utf-8">
    <title>Responsive Image Example</title>
  </head>
  <body>
    <img
      src="pic-800.jpg"
      srcset="pic-400.jpg 400w, pic-800.jpg 800w"
      sizes="(min-width: 600px) 50vw, 100vw"
      alt="A picture demonstrating valid sizes usage">
  </body>
</html>

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