The classification and application of pseudo-class selectors

Pseudo-class selectors are powerful tools in CSS that allow precise selection and styling of elements based on their state, position, or other dynamic conditions. They extend the functionality of basic selectors, making interface interactions more flexible. Pseudo-class selectors can be categorized into state pseudo-classes, structural pseudo-classes, form pseudo-classes, and others. State pseudo-classes respond to user actions like hover, active, focus, etc. Structural pseudo-classes select elements based on their position in the DOM hierarchy, such as the first child or alternating row colors. Form pseudo-classes target form control states like checked, disabled, or validation status. Combining pseudo-classes enables complex selection logic. Dynamic pseudo-classes require performance considerations, especially on mobile devices, and some may need prefixes or alternative solutions for browser compatibility. Pseudo-classes are often paired with pseudo-elements to achieve advanced effects like drop caps or tooltips.

Various forms of attribute selectors

CSS attribute selectors, enclosed in square brackets, enable element selection through basic existence checks, exact value matching, substring containment matching, prefix or suffix matching, and space or hyphen-separated list matching, among other forms. They can be combined with pseudo-classes to achieve more complex selection rules. Attribute selectors are particularly useful for targeting elements with specific attributes or values, such as link types or form controls. When using them, performance considerations are essential—avoid overly complex matching patterns. Simple attribute selectors perform comparably to class selectors, and their judicious use can significantly enhance styling flexibility.

Basic selectors (element, class, ID)

CSS selectors are the core part of style rules. The basic selectors include element, class, and ID selectors. Element selectors directly use HTML tag names to match all elements of that type in the document. Class selectors begin with a dot (.) and apply to any HTML element with the corresponding `class` attribute. An element can have multiple classes. ID selectors start with a hash (#) and select elements with a specific `id` attribute. IDs should be unique within a document. These selectors can be combined to create more specific selection rules. When multiple selectors target the same element, CSS follows a specificity hierarchy to determine which style is applied. In practice, it's best to use class selectors and keep selectors as simple as possible. Basic selectors are well-supported across all modern browsers.

The basics of the CSS box model

The CSS box model is a core concept in web layout, treating each HTML element as a rectangular box composed of content, padding, border, and margin. In the standard box model, width and height refer only to the content dimensions, while the alternative border-box model includes content, padding, and border. Margins have unique behaviors like collapsing and negative values. The box model integrates with layout systems like Flexbox and Grid, and modern browser tools allow intuitive debugging. Common issues include borders affecting layout and padding causing overflow, which can be resolved with box-sizing. Different elements, such as block-level and inline, exhibit distinct box model behaviors, and positioning methods also influence them. Complex box model calculations may impact performance, so using transform for optimization is recommended.

The CSS unit system

The CSS unit system is an essential part of styling design, including absolute units, relative units, viewport units, and functional units. Absolute units like px and pt are fixed and suitable for precise control. Relative units like em and rem are context-based, offering flexibility for responsive design. Viewport units such as vw and vh are relative to viewport dimensions, ideal for full-screen layouts. Functional units like calc and clamp support dynamic calculations. Color units include hexadecimal, RGB, and HSL. Angle and time units are used for animations and transformations. Other specialized units like fr and ch are suited for specific scenarios. Each unit has unique characteristics and can be flexibly chosen based on requirements.

The comment method in CSS

CSS comments use slashes and asterisks to wrap content, forming block comments with no single-line comment syntax. Comments can span multiple lines. Common uses include code explanations, grouping, and temporarily disabling styles. Advanced usage includes conditional comments, documentation comments, and code tagging. Preprocessors support single-line comments, but they do not appear in the compiled output. Best practices recommend keeping comments concise and informative, regularly cleaning up unused comments, and maintaining a consistent style. Good commenting habits improve code maintainability, but CSS comments cannot be nested. Comment symbols have the highest priority and ignore all internal content.

The priority calculation rules of CSS

CSS specificity determines style application rules through a weighting system, where higher-weighted styles override lower-weighted ones. Specificity is calculated based on selector types and their quantities, not code order. The specificity weight consists of four parts, ranked from highest to lowest: `!important` declarations, inline styles, ID selectors, class/attribute/pseudo-class selectors, and element/pseudo-element selectors. The weight is calculated in a digit-like manner by counting the number of each selector type. `!important` overrides all other rules but should be used sparingly. Inline styles have very high priority, while universal selectors and inherited styles have the lowest. When complex selectors are combined, their weights are summed. Pseudo-classes, pseudo-elements, and attribute selectors each contribute differently to specificity, and selector combinations affect the final weight. In real projects, specificity conflicts are common, so over-reliance on high-specificity selectors should be avoided. During debugging, browser developer tools can inspect applied styles, showing the computed specificity weight. Modern CSS methodologies like BEM manage specificity through naming conventions. Media queries do not affect selector specificity but conditionally apply styles. CSS variables follow standard specificity rules, and in Shadow DOM, external styles typically don’t affect internal components. Animation styles override regular styles, and user-agent styles have the lowest priority. When writing CSS, a balance must be struck between selector performance and specificity. Dynamic class names follow the same specificity rules. Future CSS features like `:is()` and `:where()` will alter specificity calculations.

The cascading and inheritance mechanisms of CSS

The cascading and inheritance mechanisms in CSS are core features of style sheets. The cascading mechanism determines the priority order when multiple style rules apply to the same element, based on three factors: origin, importance, selector specificity, and order of appearance. Selector specificity consists of four parts: inline styles, ID selectors, class/attribute/pseudo-class selectors, and element/pseudo-element selectors. The inheritance mechanism refers to certain CSS properties being automatically passed from parent elements to child elements. Common inheritable properties include text-related and list-related properties. CSS provides four keywords—`inherit`, `initial`, `unset`, and `revert`—to control inheritance. The stacking context affects the display order of elements along the z-axis, and its creation conditions include the root element and specific positioning properties. In practical applications, issues such as the misuse of `!important` and side effects of wildcard selectors should be noted. For performance optimization, complex selectors and overly long inheritance chains should be avoided. Modern CSS introduces variables and `@layer` rules, altering traditional cascading approaches. Browser developer tools can assist in debugging styles, while cross-browser compatibility requires special handling for form elements and pseudo-element differences.

The basic classification of CSS selectors

CSS selectors are the core component of style sheet languages, used to precisely target HTML document elements and apply style rules. The foundational selectors, including element selectors, class selectors, ID selectors, and universal selectors, form the cornerstone of the CSS selector system. Attribute selectors enable precise matching based on element attributes, supporting various matching patterns. Pseudo-class selectors dynamically target elements based on their state or document structure, while pseudo-element selectors create virtual elements for specialized styling. Combinator selectors establish element relationship chains using connectors, encompassing descendant combinators, child combinators, adjacent sibling combinators, and general sibling combinators. Selector priority is determined by specificity calculations to resolve application order. Efficient selector writing requires avoiding deep nesting, prioritizing class selectors, and limiting the scope of universal selectors. Modern CSS introduces new selectors such as logical combination pseudo-classes, direction-aware pseudo-classes, and scoping selectors.

The three ways to introduce CSS

CSS introduction methods mainly include inline styles, internal style sheets, and external style sheets. Inline styles are written directly in the `style` attribute of HTML elements, with the highest priority but poor maintainability, making them suitable for temporary debugging. Internal style sheets are embedded in HTML documents via the `<style>` tag, support selector reuse, and are ideal for single-page projects. External style sheets are imported via the `<link>` tag as standalone CSS files, support caching and multi-page reuse, and are the preferred choice for production environments. Each method has its applicable scenarios, and in practice, they are often combined. Modern front-end tools also support CSS preprocessors and modular development. Additionally, attention should be paid to the impact of different introduction methods on browser rendering performance. Large-scale projects should establish standards to ensure code maintainability.