Use srcset and sizes to improve image performance and user experience on responsive websites. 1. srcset provides multiple pictures with different sizes or resolutions, such as image-small.jpg 480w or image-2x.jpg 2x; 2. sizes informs the browser of the display width of the pictures under different viewports, such as (max-width: 600px) 100vw; 3. Common errors include ignoring the src attributes, inconsistent units, listing too many sizes, etc. These problems should be avoided to ensure correct loading and layout.
When building responsive websites, handling images properly is cruel for performance and user experience. The srcset
and sizes
attributes in HTML help browsers choose the best image to load based on screen size and resolution. It might look complicated at first, but once you understand how they work together, it becomes a powerful tool.

What srcset
Does
The srcset
attribute gives the browser a list of image files and their sizes or resolutions. This lets the browser pick the most appropriate one depending on the device. For example:
<img src="/static/imghw/default1.png" data-src="image-small.jpg" class="lazy" srcset="image-small.jpg 480w, image-medium.jpg 800w, image-large.jpg 1200w" alt="Responsive image">
Here, each image file is followed by a width descriptor ( 480w
, 800w
, etc.). The browser uses these to determine which image fits best based on the current viewport.

You can also include resolution describes like 2x
for high-DPI screens (like Retina displays):
<img src="/static/imghw/default1.png" data-src="image.jpg" class="lazy" srcset="image.jpg 1x, image-2x.jpg 2x" alt="High-res image">
In short, srcset
gives options — but it needs help to decide which one to pick.

How sizes
Helps the Browser Choose
The sizes
attribute tells the browser how big the image will appear on the page at different viewport sizes. It uses media query-like syntax to define image width in relation to the screen.
Here's an example:
<img src="/static/imghw/default1.png" data-src="image-small.jpg" class="lazy" srcset="image-small.jpg 480w, image-medium.jpg 800w, image-large.jpg 1200w" sizes="(max-width: 600px) 100vw, (max-width: 900px) 50vw, 33vw" alt="Responsive image">
Breaking it down:
- On screens narrower than 600px, the image will take up the full width (
100vw
). - Between 600px and 900px, it will be half the width (
50vw
). - On wider screens, it will be about a third of the width (
33vw
).
With this info, the browser can estimate how big the image will be on screen and choose the most suitable image from the srcset
.
Common Mistakes and Tips
When using srcset
and sizes
, it's easy to make mistakes that lead to poor performance or broken images. Here are some common issues and how to avoid them:
- Don't forget the
src
attribute. It acts as a fallback for browsers that don't supportsrcset
. - Use consistent units in
sizes
. Stick withvw
,%
, orpx
, and make sure they match the layout. - Test different viewport sizes. Use browser dev tools to see which image is loaded at different screen widths.
- Name your images clearly. Something like
image-480w.jpg
makes it easier to manage and understand. - Don't list every possible size. Aim for 3–4 main sizes (small, medium, large) to keep things manageable.
Also, remember that sizes
doesn't change the layout — it only tells the browser how much space the image will take. The actual layout should be handled with CSS.
Wrapping It Up
Using srcset
and sizes
together helps deliver the right image for each device — not too big, not too small. It's not overly complicated once you get the hang of how the browser uses these attributes. Just define your image options in srcset
, describe how big it will be on screen with sizes
, and make sure your values match your layout.
That's basically it.
The above is the detailed content of HTML `srcset` and `sizes` Explained for Responsive Images. For more information, please follow other related articles on the PHP Chinese website!

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