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Checkpoint: you can now connect the markup for responsive images and image sources to a browser behavior and a concrete debugging step.
Source-reading note: the exact semantics here come from the HTML element/attribute you chose, not its CSS appearance.
A useful way to learn this is to start with the browser's observable behavior, not the tag list. In this lesson, the focus is srcset, sizes, intrinsic dimensions, and responsive selection. That matters because HTML is interpreted by a parser and then exposed to browser features such as navigation, accessibility tooling, form controls, media handling, and script APIs. A correct-looking screenshot can still hide a poor document model.
Start with the smallest meaningful example. The working snippet demonstrates the core syntax, but the important part is what each piece contributes: the element supplies semantics or behavior, attributes refine that meaning, and surrounding structure gives the browser context. Don't add markup just because a framework or copied example happens to include it.
<img src="hero-800.jpg" srcset="hero-400.jpg 400w, hero-800.jpg 800w" sizes="(max-width: 700px) 100vw, 800px" alt="...">
Now put the same idea under pressure. A realistic failure is: Let the browser choose a reasonable resource instead of shipping desktop-sized images to every device. The browser may recover, ignore an invalid combination, or leave the user with a control that appears visually fine but behaves differently than expected. That is why source inspection and keyboard testing belong in the workflow.
<img src="hero.jpg" alt="" width="1200"><!-- Empty alt is correct only when the image is decorative. -->
Don't confuse browser-side convenience with a security boundary. The markup in responsive images and image sources should improve behavior, but authoritative validation and policy belong at the trust boundary that processes the request.
You can now explain the browser behavior behind srcset, sizes, intrinsic dimensions, and responsive selection, build the markup deliberately, and diagnose the most common failure mode instead of guessing.
Field tip: for responsive images and image sources, test both the ordinary path and the first case that makes your assumptions false.
Finish the lesson once you have worked through the material. This awards ★ 20 XP.