How to Add a Poster Image to an HTML5 Video? (The poster attribute)

The main reason for the failure of Modernizr detection is that the script is not successfully loaded or executed at an improper time. It is necessary to ensure that it is loaded synchronously, avoids CSP interception, and is executed before DOM construction. As an alternative, it is preferable to use CSS@supports and native API to detect empty scripts.

The template tag itself does not render and must be manually cloned and inserted. Template is a lazy container of HTML5. The browser will parse it but completely skip rendering and script execution. If you write Hello directly, nothing will appear on the page - this is not a bug, it is the design. To make it "alive", you must use JavaScript to extract the content, clone it, and then hang it on the DOM. A common mistake is to directly obtain document.querySelector('template').content and then try to appendChild. The result is an error or no response: because the content is a Docu

The correct way to write it is href="tel: 8613812345678". All non-numeric characters need to be cleared (only and numbers are retained). Mainland China numbers must be prefixed with 86. Extension numbers use;ext= format, and target="_blank" is disabled.

Autocomplete="off" sometimes does not take effect because modern browsers (such as Chrome ≥ 80) actively ignore it to ensure the password manager experience; to be truly effective, it needs to be combined with strategies such as semantic values ​​(such as new-password), avoiding sensitive names, and dynamically generated attributes.

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.

© and © have the same effect. The former is a named entity and is easy to read, while the latter is a decimal digital entity and has more stable compatibility. It is necessary to avoid the problems of CSS hiding, JS unescapement and missing semantics.