zes - Web APIs | MDN

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Selecting an image to fit window width

In this example, a blog-like layout is created, displaying some text and an image for which three size points are specified, depending on the width of the window. Three versions of the image are also available, with their widths specified. The browser takes all of this information and selects an image and width that best meets the specified values.

How exactly the images are used may depend upon the browser and the pixel density of the user's display.

Buttons at the bottom of the example let you actually modify the sizes property slightly, switching the largest of the three widths for the image between 40em and 50em.

HTML

html<article> <h1>An amazing headline</h1> <div class="test"></div> <p> This is even more amazing content text. It's really spectacular. And fascinating. Oh, it's also clever and witty. Award-winning stuff, I'm sure. </p> <img src="new-york-skyline-wide.jpg" srcset=" new-york-skyline-wide.jpg 3724w, new-york-skyline-4by3.jpg 1961w, new-york-skyline-tall.jpg 1060w " sizes="(50em <= width <= 60em) 50em, (30em <= width < 50em) 30em, (width < 30em) 20em" alt="The New York City skyline on a beautiful day, with the One World Trade Center building in the middle." /> <p> Then there's even more amazing stuff to say down here. Can you believe it? I sure can't. </p> <button id="break40">Last Width: 40em</button> <button id="break50">Last Width: 50em</button> </article>

CSS

cssarticle { margin: 1em; max-width: 60em; min-width: 20em; border: 4em solid #880e4f; border-radius: 7em; padding: 1.5em; font: 16px "Open Sans", "Verdana", "Helvetica", "Arial", sans-serif; } article img { display: block; max-width: 100%; border: 1px solid #888888; box-shadow: 0 0.5em 0.3em #888888; margin-bottom: 1.25em; }

JavaScript

The JavaScript code handles the two buttons that let you toggle the third width option between 40em and 50em; this is done by handling the click event, using the JavaScript string replace() method to replace the relevant portion of the sizes string.

jsconst image = document.querySelector("article img"); const break40 = document.getElementById("break40"); const break50 = document.getElementById("break50"); break40.addEventListener( "click", () => (image.sizes = image.sizes.replace(/50em,/, "40em,")), ); break50.addEventListener( "click", () => (image.sizes = image.sizes.replace(/40em,/, "50em,")), );

Result

The page is best viewed in its own window, so you can adjust the sizes fully.

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