HTML Responsive Images with Sizes and Container-Dependent Layouts

A photo, chart, or recording can explain an idea faster than a paragraph, but only if people can actually use it.

What You Need to Know

The sizes attribute describes how wide an image will be displayed in different layouts. It works with width-based srcset candidates so the browser can choose a suitable file before the final layout is drawn.

Try the Example

Take a look at the example. We’ll explain the important lines right below it.

<img src="images/city-800.jpg"
     srcset="images/city-400.jpg 400w, images/city-800.jpg 800w, images/city-1200.jpg 1200w"
     sizes="(max-width: 600px) 100vw, 50vw"
     alt="City buildings at sunset" width="800" height="450">

Let’s Read the Code

sizes: Sizes tells the browser the expected display width when using width-descriptor srcset.
<img>: The img displays a picture; its alt is the text alternative.
src: Src identifies a resource path for images, frames, or media.
srcset: Srcset offers image candidates for different displays or layout sizes.

Read the named elements and attributes together. They explain how the browser connects this feature to the rest of the page.

What Happens in the Browser?

The paths shown here refer to files in your own example project. Put real images and recordings in those locations before expecting them to load.

Which picture loads? With width descriptors, sizes describes the expected display width and srcset offers candidates. The browser can also consider screen density and network conditions. Inspect the selected source instead of assuming every screen receives the same file.

Your Turn

Resize the viewport and inspect the chosen image source in Developer Tools. Do not assume the browser always downloads the smallest file.

Common Beginner Mistakes

Leaving out real media files or understandable text alternatives.
Always check keyboard access, readable alternatives, and any missing files; a perfect player layout does not guarantee accessible content.

Key Takeaways

The sizes attribute describes how wide an image will be displayed in different layouts.
sizes: Sizes tells the browser the expected display width when using width-descriptor srcset.
If the resource cannot load, users still need enough text to understand what was meant to be there.