The Tool Desk
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How srcset and sizes work together
srcset lists image candidates and tells the browser each candidate’s intrinsic pixel width. The sizes attribute describes the expected rendered width—the layout slot—for that image. Using the slot hint and the display’s pixel density, the browser can choose a candidate; it retains discretion over the final selection.
For example, if an image fills the viewport on narrow screens and takes half the viewport on wider screens, the markup might look like this:
<img
srcset="photo-480.jpg 480w, photo-800.jpg 800w"
sizes="(max-width: 600px) 100vw, 50vw"
src="photo-800.jpg"
width="1200"
height="800"
alt="A description of the image"
>The numbers and breakpoints here are illustrative, not performance measurements. Replace them with candidates you actually provide and slot widths that match your CSS. The src supplies a default image reference; keep it as a fallback.
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Width descriptors for layout-dependent images
Descriptors such as 480w and 800w state the pixel width of each candidate file. Pair this width-descriptor form of srcset with sizes when the rendered image width changes with the layout. The browser can use the declared slot width and device pixel ratio when choosing among candidates.
Density descriptors for fixed-size images
If an image has a fixed rendered size but needs alternatives for displays with different pixel densities, use density descriptors such as 1x and 2x. This model does not use sizes. Do not mix density descriptors and width descriptors as though they described the same selection model: use width descriptors with sizes for layout-dependent widths.
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When to use <picture> instead
Use srcset on an <img> when the candidates show the same image content and crop at different resolutions or widths. Use <picture> when the browser should select a conditional alternative source—for example, a different crop for a layout or a source in a particular format. Keep an <img> inside <picture>; it remains the image element and provides the fallback.
| Need | Use | Key detail |
|---|---|---|
| Same content and crop, different widths for a responsive layout | <img srcset> with width descriptors and sizes |
Make the slot widths in sizes reflect the actual CSS layout. |
| Same content and crop, fixed rendered size, different pixel densities | <img srcset> with density descriptors |
Use descriptors such as 1x and 2x; omit sizes. |
| Different crop depending on layout, or a conditional format source | <picture> with <source> elements and a nested <img> |
The nested <img> remains the image element and fallback. |
Make image selection match the page layout
Write sizes from the rendered slot
For width candidates, sizes is a list of media-condition and length pairs. The browser uses the first matching condition as its slot-width hint. Describe the width the image occupies in the page, not the dimensions of a candidate file. If your CSS layout changes, revisit sizes as well; a mismatched hint can lead the browser to select a candidate that does not fit the intended slot efficiently.
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The sample uses (max-width: 600px) 100vw, 50vw: up to 600 pixels of viewport width, it hints that the image occupies the full viewport width; otherwise it hints that the slot is half the viewport width. Those values are only correct if the page’s CSS actually behaves that way.
Include intrinsic dimensions when known
Set the image’s intrinsic width and height attributes when you know them, then use CSS to control its responsive rendered size. The browser can use the intrinsic ratio to reserve space as the image loads, helping reduce layout shifts even when CSS changes the displayed width.
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Implement and verify a responsive image
- Identify the image behavior. Decide whether the crop and content stay the same, whether the rendered width varies with layout, and whether you need conditional formats or a different crop.
- Prepare appropriate candidates. For a layout-dependent image, provide files at known widths and list those widths with
wdescriptors. For a fixed-size image with density variants, use density descriptors instead. - Describe the slot accurately. For width candidates, write
sizesconditions that mirror the CSS layout’s rendered slot widths. Put the most specific matching conditions first. - Keep a fallback and dimensions. Set
srcto a usable default image and include intrinsicwidthandheightwhen known. For art direction or conditional formats, place the sources and fallback image in<picture>. - Check the result across layouts. Compare the rendered slot with the conditions in
sizes, and check that the listed candidates correspond to the files actually served. Selection can vary because the browser has discretion; do not assume it must always choose a particular candidate.
Performance expectations and trade-offs
Responsive candidates let the browser choose from files suited to the intended slot and display density, rather than requiring one image file for every situation. They do not guarantee a fixed reduction in bytes. The outcome depends on the source files, slot sizes, display density, compression, browser choice, and delivery conditions. Choose candidates and compression appropriate to your images, and judge the result in the context of the page’s actual layout.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Troubleshooting common responsive-image problems
| Symptom | Likely cause | What to check or change |
|---|---|---|
| The browser downloads a candidate that seems too large or too small for the layout. | The sizes hint may not describe the CSS slot, or the candidate set may not include a suitable width. |
Compare each matching sizes condition and length with the rendered slot, then verify the intrinsic pixel widths declared by the w descriptors. |
| The image looks soft on a dense display. | The available candidates may not provide an appropriate resolution for the rendered size and pixel density. | Check the candidate dimensions and, for layout-dependent images, use width descriptors with an accurate sizes value. For a fixed rendered size, consider density descriptors. |
| A layout change shows the wrong crop. | srcset on its own selects among candidates of the same image; it is not the pattern for conditional art direction. |
Use <picture> with conditional <source> elements and retain the nested <img> fallback. |
| The page shifts as the image loads. | The browser may not have intrinsic dimensions to use when reserving space. | Supply the known intrinsic width and height attributes and let CSS control the responsive rendered size. |
| The fallback image is missing or unsuitable. | The src reference may be absent or point to an unavailable default. |
Keep a valid src on the <img>; inside <picture>, make sure the nested <img> remains present. |
Or skip the browser setup
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