What is the semantic meaning of the footer element in HTML5?

Why can't the tag directly display the upload progress? It is a read-only visual component. It does not listen to network requests and is not automatically bound to the upload process of XMLHttpRequest or fetch. If you put it in and don't update the value manually, it will always stop at 0%. What really drives it is the event monitoring in the upload logic you write yourself. A common mistake is to only monitor onload (upload completed) but miss upload.onprogress. XMLHttpRequest (not fetch) must be used to obtain real-time upload progress, because fetch does not expose the max attribute of the event in the upload phase and must be set to the file size (file.size

The title attribute is not a tooltip component, but an accessibility prompt mechanism implemented by the browser. The behavior, style, and interaction are uncontrollable and are only suitable for simple scenarios such as pure information supplement.

Flexbox is the most stable for centered images. The key is to set display:flex and align-items:center in the parent container and specify the height; using place-items:center for Grid is more concise; absolute positioning requires top:50% with transform:translateY(-50%); vertical-align is invalid for block-level centering.

ApplicationCache has been completely abandoned and will be removed from Chrome 61, Firefox 72, and Safari 11.1. ServiceWorker must be used instead; the latter requires HTTPS, manual registration and cache control, and the path, scope, and life cycle must match exactly.

required only verifies that it is not empty, not the format; type="email" or pattern must be used together; native verification is only triggered when submitting, not real-time; checkbox/radio/select/textarea has special behavior; the server must re-verify and clean empty values.

Use and implement native folding area. HTML5 natively supports folding and expansion. It can work without JS. It is a container and a click area. The browser adds a small triangle by default. Click it to expand/collapse the content. A common mistake is to write a closing tag (for example), which must have a start and end tag, and must be the first child element of , otherwise the folding logic will fail. The default is the collapsed state; add the open attribute to expand it by default: you can put text, icons, and even text inside, but don't nest another one - some browsers have inconsistent behavior and do not support IE. Edge79, Chrome12, Firefox49, and Safari6 have good support.

The element does not render by default, does not occupy the layout, and is not recognized by screen readers. It must explicitly set the open attribute or call showModal()/show() to activate; to close, you need to call close(), click mask or press Esc to monitor manually; the old version of Safari needs to be polyfilled or downgraded; form submission will not automatically close and must be intercepted and manually controlled.

What are the values ​​of the preload attribute, and what are their effects? The preload of HTML5 is a suggestive attribute. The browser is not forced to comply with it, but it will affect the initial resource loading strategy. It has only three legal values: auto, metadata, none. auto: It is recommended that the browser downloads the entire media file (including audio and video data) as soon as possible, which is suitable for scenarios where users are likely to play it, such as carousel videos on the homepage; but it will waste bandwidth, especially on mobile terminals or under weak networks. metadata: only prefetch meta information such as duration, size, frame rate, cover image, etc., without loading the actual audio and video frames, which is a safe choice in most cases none: explicitly tell the browser "don't load it yet" and wait until the user triggers playback (such as clicking) before starting it