Hey there! Let's unpack your questions about the <picture> element in detail—this stuff is super useful for responsive image handling, so I’m glad you’re diving into it.
Think of the <picture> element as a "smart container" that lets you serve tailored image sources based on the browser’s capabilities or the user’s viewport. Here’s the step-by-step breakdown:
Browsers parse the <picture> element from top to bottom, checking each <source> tag in sequence.
For each <source>, it first validates if the media attribute (if present) matches the current viewport conditions (like max-width, screen orientation, etc.). If there’s a type attribute, it also checks if the browser supports that image format (e.g., image/webp).
The first <source> that meets all the browser’s criteria gets prioritized: the browser loads the image specified in its srcset (or src) attribute.
If none of the <source> tags are a match, the browser falls back to loading the <img> tag that should always be included inside <picture>—this acts as a safety net for older browsers or unmatched scenarios.
Key note: The order of <source> tags matters! Browsers stop at the first matching one, so always put more specific conditions first.
Responsive bandwidth optimization: The media="(max-width: 960px)" rule ensures mobile/tablet users (with viewports ≤960px) load the smaller 800x690px image (dynamically generated via the PHP get_page_thumbnail function) instead of a larger desktop-sized image—cutting down on unnecessary data usage.
IE9 compatibility hack: IE9 doesn’t natively support the <picture> element, but it does recognize <source> tags when they’re nested inside a <video> element. The conditional comments (<!--[if IE 9]>...<![endif]-->) wrap the <source> in a hidden <video> only for IE9, tricking the browser into processing the <source> and loading the correct image, rather than ignoring it entirely.
Dynamic image sizing: The PHP get_page_thumbnail function generates a pre-resized thumbnail URL, avoiding the inefficiency of scaling a large image down on the user’s device.
Modern browsers (with native <picture> support) check <source> tags top to bottom: if the viewport is ≤960px, they load the 800x690px image from the first <source>. For wider viewports, they’ll skip this <source> and look for the next matching one (or fall back to the <img> tag).
For IE9, the conditional comment makes the <source> appear as part of a hidden <video>, so the browser treats it as a valid resource and loads the image specified in srcset.
The core responsive logic is pure HTML5—no front-end framework is required to make <picture> work.
The PHP get_page_thumbnail function is likely part of a custom CMS or PHP-based system (like WordPress, though not explicitly stated), but that’s only for generating the image URL. The image selection behavior itself relies on native browser features.
The IE9 hack uses only legacy IE conditional comments and native HTML elements, so no external frameworks or polyfills are needed for that specific compatibility fix.
No duplicate upfront loads: Browsers only load the first <source> that matches the current viewport conditions—they won’t waste bandwidth loading unused sources.
Automatic resolution switching isn’t native: The <picture> element doesn’t automatically swap images when the viewport resizes (e.g., rotating a phone or resizing a desktop window). To handle that, you’d need to add custom JavaScript to listen for viewport changes and trigger a reload of the appropriate image, but that’s not part of the native behavior.
No post-load switching by default: Once a matching image is loaded, the browser won’t switch to another source unless explicitly instructed to do so.
