What is the LVHA rule in CSS?

The LVHA rule is the strict required ordering for styling CSS anchor links: Link (:link), Visited (:visited), Hover (:hover), and Active (:active). If you break this specific order, CSS specificity rules may cause later states like hover to fail or be ignored by the browser.

Why is my ::before or ::after pseudo-element not showing up?

Your `::before` or `::after` pseudo-element is likely missing the mandatory `content` property. Even if you only want to render a decorative colored box, you must explicitly declare `content: ''` (an empty string) to force the CSS engine to render the virtual node.

What is the difference between a pseudo-class and a pseudo-element?

A pseudo-class (using a single colon `:`) targets the *state* or DOM position of an existing HTML element (like `:hover` or `:first-child`). A pseudo-element (using double colons `::`) targets a *specific part* of an element or creates an entirely new virtual element in the DOM (like `::before` or `::selection`).

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CSS Pseudo Elements & Classes: The Complete Guide

Master CSS Pseudo-classes and Pseudo-elements. Learn to handle user interactions with hover and focus, inject virtual elements with ::before and ::after, and master the LVHA rule.

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Pseudo Logic Engines

State and Virtual Logic. Extend CSS selectors to react to transient user states, specific DOM indexing, and virtually injected data nodes.

Pseudo-classes and pseudo-elements expand CSS from a static styling tool into a dynamic logic engine. They allow you to style elements based on user interaction, their specific parts, and their position in the document.

1Interactive States (Pseudo-classes)

Pseudo-classes (indicated by a single colon :) represent a specific state of an element.

→`:hover`: Triggers when a mouse pointer enters the element's area. Essential for interactive buttons and links.
→`:focus`: Activates when an element is selected (via click or Tab key). This is critical for accessibility so keyboard users know exactly where they are on the page.
→`:active`: The 'pressed' state. Triggers while the user is actively clicking or pressing the element.
→LVHA Rule: Links must be styled in the exact order: Link, Visited, Hover, Active. This is a factual CSS cascading rule; breaking this order causes later states to fail.

2Structural Logic (Pseudo-elements)

Pseudo-elements (indicated by double colons ::) target specific sub-parts or create virtual ones natively in CSS.

→`::before` & `::after`: These create 'virtual' children inside an element. They strictly require a content property (even if empty content: '') to render. They are perfect for injecting decorative icons, lines, or quotes without bloating the HTML.
→`::selection`: Allows you to customize the highlight color when a user selects text with their cursor.
→`::placeholder`: Targets the hint text inside input fields, allowing separate styling from the user's typed text.