The semantic meaning of the document

Semantic HTML involves selecting appropriate tags to accurately describe the meaning of content rather than just controlling its appearance. HTML5 introduced semantic tags like `<article>`, `<section>`, etc., to create a clear document structure, enabling both machines and developers to understand the document hierarchy. Inline elements such as `<time>`, `<mark>`, and `<strong>` express micro-semantics for text content. Forms improve accessibility through `<fieldset>`, `<label>`, and ARIA attributes. Multimedia leverages `<figure>`, `<figcaption>`, and `<track>` elements to provide contextual support. Data tables optimize structure with `<scope>`, `<caption>`, and zoning. Semantic markup also enhances SEO by using Schema.org microdata to improve search result displays. For older browsers, progressive enhancement strategies ensure compatibility. CSS can reduce code volume by leveraging semantic selectors. Tools can quantitatively evaluate semantic quality, with excellent sites typically scoring above 90 points.

The use of id and class attributes

In HTML, `id` and `class` are core global attributes. The `id` is used to uniquely identify an element, offering high specificity and facilitating direct manipulation via JavaScript. The `class` can be reused, supports combining multiple class names, and is suitable for style reuse and state management. The `id` is ideal for major page structures or unique elements, while `class` is better for reusable components and batch operations. Naming should be semantic, avoiding vague terms. Modern frameworks extend the use of `class`, such as React's `className` and Vue's dynamic binding. Proper use of `id` and `class` also enhances accessibility—associating form elements or indicating state changes. Performance-wise, avoid excessive use of `id` selectors, as batch operations with `class` are more efficient. In responsive design, `class` is commonly used to implement layout changes.

The difference between div and span

The `div` and `span` are commonly used container elements in HTML but have distinct characteristics. The `div` is a block-level element that occupies its own line and automatically fills the width of its parent container, making it suitable for layout structures like headers, footers, and other large section divisions. The `span` is an inline element, occupying only the space required by its content without forcing line breaks, making it ideal for styling text fragments or adding specific behaviors. A `div` can contain other block-level and inline elements, while a `span` should typically only hold text or other inline elements. There are also differences in CSS styling: properties like `text-align` work effectively on `div`, whereas `vertical-align` is commonly used on `span`. In responsive design, `div` often serves as a flex or grid container, while `span` retains its inline nature, making it suitable for flowing text. In practical projects, `div` is frequently used to build the page skeleton, while `span` handles text-level styling and interactions. Using both appropriately results in cleaner and more structured code.

The declaration of the character set

The HTML character set declaration is crucial for ensuring browsers correctly parse and display text content. HTML5 recommends using the meta charset UTF-8 declaration, which should be placed at the very beginning of the head section. Traditional HTML4 used a more complex http-equiv method. Servers can also specify the character set via HTTP headers. UTF-8 is the most recommended encoding as it supports all Unicode characters. Form submissions and AJAX requests are also affected by the character set. Incorrect declarations may lead to garbled text or security issues. Multilingual websites must use UTF-8. Mobile devices and emails also require attention to character set declarations. Tools can help verify encoding correctness. Dynamic content and database connections must maintain consistent character set settings. In the future, UTF-8 will continue to be the web encoding standard, and as needs evolve, more comprehensive character support may emerge.

The introduction of the script (script)

In HTML, there are multiple ways to introduce JavaScript scripts, including inline scripts and external script imports. Inline scripts are written directly in HTML and are suitable for small code snippets, while external scripts are imported via the `src` attribute, which facilitates code reuse and maintenance. Script loading attributes like `async` and `defer` control execution timing. Dynamic script loading enables on-demand loading. Modular scripts support ES6 modules via `type="module"`. Performance optimizations include inlining critical scripts and deferring non-critical ones. Compatibility handling involves ES5 transpilation and feature detection. Security considerations include integrity checks and CSP policies. Modern frameworks like React and Vue have special script handling methods. For debugging, inspect the network panel and console error messages. Performance monitoring can measure module loading times.

The introduction of style sheets

HTML introduces CSS stylesheets in multiple ways, including inline styles, embedded `<style>` tags, and external file linking via `<link>`. Inline styles have the highest priority but are harder to maintain, while external stylesheets separate content from presentation, enabling sharing and caching. CSS supports `@import` rules, but they impact loading performance. Style priority decreases from `!important` declarations to element selectors. Media queries enable responsive design, and preloading optimizes critical CSS for performance. JavaScript can dynamically load styles. Modern frameworks offer component-level style isolation, such as Vue's `scoped` and React's CSS-in-JS. Browser compatibility must be considered—older IE versions can use conditional comments, while modern development recommends feature queries and progressive enhancement.

External resource links (link)

HTML external resource links are core elements in web development for connecting different documents and resources, primarily implemented through the `href` attribute of the `<a>` tag. Links can be categorized into absolute URLs, relative URLs, page anchors, and special protocol links. The `target` attribute controls how links open, such as `_blank` for opening in a new window, while the `rel` attribute defines document relationships, like `nofollow` to prevent search engine tracking. Special protocols include `mailto`, telephone, and maps. The `download` attribute enables file downloads. CSS can style links for different states. Accessibility practices require clear text alternatives. Security measures include `noopener` to prevent access to the original page. JavaScript can dynamically generate and manage links. Performance optimization techniques include preconnecting and preloading. Social media sharing links can include tracking parameters, and analytics parameters can be added to links. Regularly validating link effectiveness is crucial. Deep linking technology enables in-app navigation, and structured data helps search engines understand link relationships. Together, these techniques form the foundation of modern web navigation and resource connectivity.

The setting of the base URL (base)

The base URL, defined by the `<base>` tag, specifies the resolution baseline for all relative URLs in a document. It must be placed within the `<head>` section and prioritized before other URL-containing elements. A document can only have one `<base>` tag, which includes two optional attributes: `href` and `target`. The `href` defines the base URL path, while `target` specifies the default link opening behavior. When the `href` value is a directory path, it must end with a slash. Once the base URL is set, all relative paths will be resolved based on it. Different syntaxes for multi-level paths yield different results. The `target` attribute affects all links without explicitly specified targets. JavaScript can dynamically modify the base URL, but this won’t affect already loaded resources. The base URL also impacts relative paths in CSS resources. Special scenarios include handling URL fragments and protocol-relative URLs. For server-side rendering, absolute paths are recommended. Single-page applications require special handling to ensure routing configurations match the base URL. Debugging techniques include inspecting the base URL in the console. Security considerations involve avoiding user-controllable content. For performance optimization, static resources can use a dedicated base URL on a separate domain.

Page refresh and redirection

Page refresh is the process of the browser reloading the current page, which can be achieved via JavaScript or meta tags for automatic refresh but incurs performance overhead. Page redirection can be implemented in various ways, including `<a>` tags, JavaScript redirects, form submissions, and meta tag auto-redirects, with considerations for user experience, SEO impact, performance optimization, and security. Advanced redirection techniques include delayed redirects, conditional redirects, and multi-step redirects. Browser history management involves adding or replacing history entries and handling forward/back navigation. Cross-domain redirects face security restrictions, while mobile platforms require handling in-app redirects and back button behavior. Performance optimization techniques include preloading target pages, using Web Workers to prepare redirects, and adding transition animations.

Keywords and description information

Keywords and description in HTML documents are crucial for search engine optimization and user experience. Keywords are set via meta tags, but modern search engines no longer use them as ranking factors. The description directly influences search result displays and user click-through rates. A well-crafted description should accurately summarize the page content, include primary keywords, and be compelling. Modern SEO also requires consideration of social media metadata like Open Graph Protocol and Twitter Cards. Mobile optimization necessitates special viewport settings, while security-related meta tags can enhance website protection. Combining semantic HTML5 elements and structured data improves content recognition. Dynamic websites require special handling for metadata updates, and CMS systems should establish meta field management mechanisms. Proper use of meta tags effectively enhances website discoverability and user experience.