How to Build Responsive Images with srcset

This article, part of Microsoft's web development series, explores responsive images – a crucial aspect of creating websites that function flawlessly across diverse devices. It's no longer optional; it's a necessity in our multi-device world.

Key Takeaways:

• Use the srcset attribute to offer various image sources optimized for different device pixel ratios.
• Combine srcset with the sizes attribute to specify image display sizes relative to the viewport, guiding the browser to choose the best image.
• Leverage the <picture></picture> element for enhanced flexibility, enabling image selection based on media conditions (like CSS media queries) and supporting various image formats via the type attribute.
• Always include a default image using the src attribute as a fallback for browsers lacking srcset support.
• Thoroughly test responsiveness and performance across various devices and browsers using tools like those offered by Microsoft for compatibility checks and web experience optimization.

Understanding Responsive Images:

A responsive image adapts its presentation to the viewing device. "Best form" means different things:

• Displaying different image assets based on screen size (e.g., a different image for a 13.5-inch laptop versus a 5-inch phone).
• Using different images based on device resolution (or device-pixel ratio – the ratio of device pixels to CSS pixels).
• Selecting an image format (like JPEG XR) if supported by the browser, potentially for better compression.

Enabling Responsive Images:

While older scripting methods exist, they're not recommended due to potential issues with multiple downloads or missing images. The preferred approach uses:

1. srcset attribute
2. sizes attribute
3. <picture></picture> element

The srcset Attribute:

Before diving into srcset usage, let's define:

Device-Pixel Ratio: This is the number of device pixels per CSS pixel, influenced by:

1. Device pixel density (physical pixels per inch): Higher resolution devices have higher density and thus a higher device-pixel ratio at the same zoom level.
2. Browser zoom level: Higher zoom levels increase the device-pixel ratio for a given device.

Basic srcset implementation uses the x descriptor for device-pixel ratio:

<code class="language-html"><img src="/static/imghwm/default1.png" data-src="https://img.php.cn/upload/article/000/000/000/174027748422704.jpg" class="lazy" alt="How to Build Responsive Images with srcset "></code>

• 1x: space-needle.jpg for a device-pixel ratio of 1.
• 2x: space-needle-2x.jpg for a device-pixel ratio of 2.
• 3x: space-needle-hd.jpg for a device-pixel ratio of 3.

The src attribute serves as a fallback for browsers without srcset support. The w descriptor specifies image width:

<code class="language-html"><img  src="/static/imghwm/default1.png" data-src="images/space-needle.jpg" class="lazy" srcset="images/space-needle.jpg 200w, images/space-needle-2x.jpg 400w, images/space-needle-hd.jpg 600w" alt="How to Build Responsive Images with srcset" ></code>

The sizes Attribute:

To control image size based on viewport size, use the sizes attribute with the w descriptor in srcset.

Example 1 (Image at half viewport width):

<code class="language-html"><img  src="/static/imghwm/default1.png" data-src="images/space-needle.jpg" class="lazy" sizes="50vw" srcset="images/space-needle.jpg 200w, images/space-needle-2x.jpg 400w, images/space-needle-hd.jpg 600w" alt="How to Build Responsive Images with srcset" ></code>

Example 2 (Responsive sizing based on viewport width):

<code class="language-html"><img  src="/static/imghwm/default1.png" data-src="images/space-needle.jpg" class="lazy" sizes="(max-width: 40em) 100vw, 50vw" srcset="images/space-needle.jpg 200w, images/space-needle-2x.jpg 400w, images/space-needle-hd.jpg 600w" alt="How to Build Responsive Images with srcset" ></code>

The <picture></picture> Element:

The <picture></picture> element allows for image selection based on rendered size and browser capabilities. It contains <source></source> elements with srcset, sizes, media, and type attributes. An <img src="/static/imghwm/default1.png" data-src="https://img.php.cn/upload/article/000/000/000/174027748575415.png" class="lazy" alt="How to Build Responsive Images with srcset ">

(Note: Image sources and sizes are illustrative.)

Conclusion:

Responsive images enhance user experience across devices. Implement them for optimal website performance. Further resources and tools from Microsoft are available for cross-browser testing and web development best practices. A detailed FAQ section follows, addressing common questions about srcset implementation.

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