When the “srcset” attribute has any image candidate string with a width descriptor, the “sizes” attribute must also be specified.

The srcset attribute lets you provide multiple image sources so the browser can select the most appropriate one. There are two types of descriptors you can use in srcset: pixel density descriptors (e.g., 1x, 2x) and width descriptors (e.g., 400w, 800w). When you use pixel density descriptors, the browser already knows the relationship between each source — it simply picks the one matching the device’s pixel ratio. But width descriptors work differently. They tell the browser the intrinsic pixel width of each image file, and the browser then needs to know how wide the image will actually be rendered on screen to calculate which file is the best fit. That’s exactly what the sizes attribute provides.

The sizes attribute accepts a comma-separated list of media conditions paired with length values, plus a default length. For example, sizes="(max-width: 600px) 100vw, 50vw" tells the browser: “If the viewport is 600px wide or less, this image will occupy 100% of the viewport width; otherwise, it will occupy 50%.” Armed with this information and the width descriptors in srcset, the browser can do the math and download only the most suitable image — before CSS or layout has even been calculated.

Why this matters

• Standards compliance: The HTML specification requires sizes whenever srcset uses width descriptors. Omitting it produces invalid HTML.
• Correct image selection: Without sizes, browsers fall back to assuming the image will be 100vw wide, which often leads to downloading unnecessarily large images on desktop layouts where the image is much smaller than the full viewport.
• Performance: Serving oversized images wastes bandwidth and slows page load. A proper sizes attribute ensures the browser downloads the smallest sufficient image.
• Predictable behavior: Relying on the browser’s fallback assumption (100vw) makes your responsive images behave inconsistently and defeats the purpose of providing multiple candidates.

How to fix it

1. Identify every img (or source) element that uses width descriptors in srcset.
2. Determine how wide the image will be displayed at different viewport sizes. You can inspect this with your browser’s developer tools or by reviewing your CSS.
3. Add a sizes attribute that describes those widths using media conditions and CSS length values like px, vw, em, or calc() expressions.

Examples

Incorrect — missing sizes with width descriptors

<img
  src="photo-400.jpg"
  srcset="photo-400.jpg 400w, photo-800.jpg 800w, photo-1200.jpg 1200w"
  alt="A mountain landscape">

This triggers the validation error because the browser sees width descriptors (400w, 800w, 1200w) but has no sizes attribute to determine the image’s rendered width.

Correct — sizes attribute added

<img
  src="photo-400.jpg"
  srcset="photo-400.jpg 400w, photo-800.jpg 800w, photo-1200.jpg 1200w"
  sizes="(max-width: 600px) 100vw, (max-width: 1000px) 50vw, 600px"
  alt="A mountain landscape">

Here, the sizes attribute tells the browser:

• On viewports up to 600px wide, the image fills 100% of the viewport.
• On viewports between 601px and 1000px, the image fills 50% of the viewport.
• On larger viewports, the image is displayed at a fixed 600px width.

Correct — pixel density descriptors (no sizes needed)

<img
  src="logo-1x.png"
  srcset="logo-1x.png 1x, logo-2x.png 2x"
  alt="Company logo">

When using pixel density descriptors (1x, 2x) instead of width descriptors, the sizes attribute is not required. The browser simply matches the descriptor to the device’s pixel ratio.

Correct — using sizes with a <picture> element

<picture>
  <source
    srcset="hero-400.webp 400w, hero-800.webp 800w"
    sizes="(max-width: 800px) 100vw, 800px"
    type="image/webp">
  <img
    src="hero-800.jpg"
    srcset="hero-400.jpg 400w, hero-800.jpg 800w"
    sizes="(max-width: 800px) 100vw, 800px"
    alt="Hero banner">
</picture>

The sizes attribute is required on both the source and img elements when either uses width descriptors in its srcset.

Learn more:

MDN Reference

MDN Web Docs: Responsive images - Guide to the srcset and sizes attributes

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MDN Reference

HTML img element - sizes attribute

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MDN Reference

HTML Responsive Images

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WHATWG Spec

Images in HTML specification

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