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Use srcset with sizes to let the browser choose among differently sized versions of the same image; use <picture> when the image itself should change, such as to show a different crop or format. CSS controls how large the image appears. Responsive-image markup helps the browser choose which resource to fetch.
Responsive display and responsive image selection are different
An image can shrink to fit a phone screen using CSS, for example with max-width: 100%; height: auto;. That changes its displayed size, but it does not by itself give the browser a smaller image file to download. HTML responsive-image markup offers alternate resources so the browser can select one suited to the layout and device.
For the markup to help, you need actual image variants—such as 480-, 800-, and 1200-pixel-wide files. The browser cannot make a smaller source file available merely because CSS reduces the displayed image.
Use srcset and sizes for the same image at different widths
When the content and composition stay the same but the image occupies different CSS widths at different layouts, put width-descriptor candidates in srcset and describe the expected rendered slot in sizes. The browser uses those inputs alongside device conditions to choose a candidate. sizes is a selection hint; it does not set the image’s CSS width. See the WHATWG HTML Standard’s image section and MDN’s responsive images guide.
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<img
src="photo-800.jpg"
srcset="photo-480.jpg 480w, photo-800.jpg 800w, photo-1200.jpg 1200w"
sizes="(max-width: 600px) 100vw, 800px"
width="800"
height="600"
alt="Describe the meaningful content of the image"
>This example assumes the image fills the viewport up to 600 CSS pixels, then occupies an 800 CSS-pixel slot. Change sizes to match the actual layout. The 480w, 800w, and 1200w descriptors must correspond to the pixel widths of the respective files; they are not arbitrary labels. The intrinsic width and height provide the image’s dimensions so the browser can reserve an appropriately proportioned box while it loads.
Make sizes describe the slot the image actually occupies
For a two-column layout, a slot might be roughly half the content area rather than the full viewport. A matching sizes value could describe that relationship, for example (max-width: 600px) 100vw, 50vw, if the image really fills the viewport on narrow screens and half the viewport on wider screens. Account for page gutters and fixed-width containers when needed; sizes should describe the image’s expected slot, not simply repeat a convenient breakpoint.
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If the value understates the slot, the browser may select an image that is too small; if it overstates it, the browser may fetch a larger resource than needed. The browser makes the final candidate choice, so the markup supplies useful inputs rather than guaranteeing a particular file on every device.
Use picture when the crop or source should change
Choose <picture> for art direction: the image’s content, crop, or composition changes for a layout. Put the more specific matching <source> first, provide its condition, and keep an <img> fallback. The WHATWG standard describes this pattern for images that vary in content, including a smaller crop of a larger image.
Rank #3
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<picture>
<source media="(max-width: 600px)" srcset="portrait-crop.jpg">
<img src="landscape.jpg" width="1200" height="700" alt="Describe the scene">
</picture>This minimal example swaps a portrait crop in at the narrow breakpoint and uses the landscape image otherwise. For a real layout, choose the breakpoint and image dimensions to fit the design. The fallback <img> remains necessary and carries the alternative text.
Offer format alternatives with picture and type
The <picture> and <source> model can also provide format alternatives using a source’s type attribute, followed by an <img> fallback. This lets the browser select a supplied source it can use. Which format is suitable depends on file size, visual quality, encoding workflow, and the browsers and other clients the site intends to support; there is no universally best format established here.
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<picture>
<source type="image/avif" srcset="photo.avif">
<source type="image/webp" srcset="photo.webp">
<img src="photo.jpg" width="1200" height="800" alt="Describe the meaningful content">
</picture>Use files that represent the same content and intended composition when offering format alternatives. A format source is not a replacement for the fallback image.
Choose width or density descriptors based on the layout
| Need | Use | Authoring concern |
|---|---|---|
| Same image content, several resource widths | srcset width descriptors with sizes |
Match descriptors to real file widths and make sizes reflect the rendered slot. |
| Same content at a known rendered size, with variants for pixel density | srcset density descriptors, such as 1x and 2x |
Use when the rendered size is known, not to describe a slot that changes with viewport width. |
| Different crop or composition at a breakpoint | <picture> with <source media> |
Choose a crop suited to the design and retain a fallback <img>. |
| Different image formats for the same image | <picture> with typed sources and an <img> fallback |
Provide a fallback and consider the formats the site intends to support. |
The WHATWG standard states: “The x descriptor is not appropriate when the rendered size of the image depends on the viewport width (viewport-based selection), but can be used together with art direction.” In practical terms, use width descriptors and sizes when the slot changes with the layout; use density descriptors when the rendered size is known.
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Keep the fallback and write useful alternative text
For <picture>, the nested <img> is the fallback when none of the provided sources is selected. Its alt text is the textual replacement for the image, so write it according to the image’s purpose in context. An informative image needs a useful description; a decorative image may need empty alternative text. The MDN picture reference and MDN img reference cover source selection and image alternatives.
Check the markup against the page layout
- Identify whether the image keeps the same content and crop across layouts. If it does, prepare resource-width variants; if its composition should change, prepare art-directed crops and use
<picture>. - Record the widths of the actual files and use those values as the
wdescriptors insrcset. - Work out the image’s expected slot width at each relevant layout and express it in
sizeswhen using width descriptors. - Set intrinsic
widthandheight, and write context-appropriatealttext on the fallback<img>. - Test the page at narrow and wide layouts and check that the crop, displayed size, and selected resources make sense for the layout.
What savings to expect—and what not to assume
Smaller candidates can avoid downloading a needlessly large source when the layout and browser selection make them appropriate, but savings depend on the available files, page layout, browser choice, and device. MDN’s instructional example gives an 800-pixel image at 128 KB and a 480-pixel image at 63 KB—a 65 KB difference between those two example files. That is an illustrative comparison, not a general benchmark or a promise of savings on every page.
Or skip the browser setup
Responsive-image markup still belongs in your page; a screenshot API does not author srcset or <picture> for you. To inspect how a public page appears at a chosen viewport without setting up a browser capture script, you can request a screenshot from ScreenshotNeo, a website screenshot API and MCP server. Its 12 device presets and custom viewport options can help you check layouts. One cURL request:
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