pseudo-elements（伪元素）

Pseudo Elements

Review CSS and JavaScript for pseudo-element best practices and View Transitions API usage.

How It Works

1. Read the specified files (or prompt user for files/pattern)
2. Check against all rules below
3. Output findings in file:line format

Rule Categories

Rules

Before/After Rules

pseudo-content-required

::before and::after require content property to render.

Fail:

.button::before {
  position: absolute;
  background: var(--gray-3);
}

Pass:

.button::before {
  content: "";
  position: absolute;
  background: var(--gray-3);
}

pseudo-over-dom-node

Use pseudo-elements for decorative content instead of extra DOM nodes.

Fail:

<button className={styles.button}>
  <span className={styles.background} /> {/* Unnecessary DOM node */}
  Click me
</button>

Pass:

<button className={styles.button}>
  Click me
</button>

.button::before {
  content: "";
  /* decorative background */
}

pseudo-position-relative-parent

Parent must have position: relative for absolute pseudo-elements.

Fail:

.button::before {
  content: "";
  position: absolute;
  inset: 0;
}
/* .button has no position */

Pass:

.button {
  position: relative;
}

.button::before {
  content: "";
  position: absolute;
  inset: 0;
}

pseudo-z-index-layering

Pseudo-elements need z-index to layer correctly with content.

Fail:

.button::before {
  content: "";
  position: absolute;
  inset: 0;
  background: var(--gray-3);
}
/* Covers button text */

Pass:

.button {
  position: relative;
  z-index: 1;
}

.button::before {
  content: "";
  position: absolute;
  inset: 0;
  background: var(--gray-3);
  z-index: -1;
}

pseudo-hit-target-expansion

Use negative inset values to expand hit targets without extra markup.

Fail:

<div className={styles.wrapper}> {/* Extra wrapper for hit target */}
  <a className={styles.link}>Link</a>
</div>

Pass:

.link {
  position: relative;
}

.link::before {
  content: "";
  position: absolute;
  inset: -8px -12px;
}

View Transitions Rules

transition-name-required

Elements participating in view transitions need view-transition-name.

Fail:

document.startViewTransition(() => {
  // No view-transition-name assigned
  targetImg.src = newSrc;
});

Pass:

sourceImg.style.viewTransitionName = "card";
document.startViewTransition(() => {
  sourceImg.style.viewTransitionName = "";
  targetImg.style.viewTransitionName = "card";
});

transition-name-unique

Each view-transition-name must be unique on the page during transition.

Fail:

.card {
  view-transition-name: card;
}
/* Multiple cards with same name */

Pass:

// Assign unique name only to transitioning element
element.style.viewTransitionName = `card-${id}`;

transition-name-cleanup

Remove view-transition-name after transition completes.

Fail:

sourceImg.style.viewTransitionName = "card";
document.startViewTransition(() => {
  targetImg.style.viewTransitionName = "card";
});
// sourceImg still has name, causes conflict on next transition

Pass:

sourceImg.style.viewTransitionName = "card";
document.startViewTransition(() => {
  sourceImg.style.viewTransitionName = "";
  targetImg.style.viewTransitionName = "card";
});

transition-over-js-library

Prefer View Transitions API over JavaScript animation libraries for page transitions.

Fail:

import { motion } from "motion/react";

function ImageLightbox() {
  return (
    <motion.img layoutId="hero" /> // JS-based shared element transition
  );
}

Pass:

function openLightbox(img: HTMLImageElement) {
  img.style.viewTransitionName = "hero";
  document.startViewTransition(() => {
    // Native browser transition
  });
}

transition-style-pseudo-elements

Style view transition pseudo-elements for custom animations.

Fail:

document.startViewTransition(() => { /* ... */ });
// Uses default crossfade

Pass:

::view-transition-group(card) {
  animation-duration: 300ms;
  animation-timing-function: cubic-bezier(0.215, 0.61, 0.355, 1);
}

Native Styling Rules

native-backdrop-styling

Use::backdrop pseudo-element for dialog/popover backgrounds.

Fail:

<>
  <div className={styles.overlay} onClick={close} />
  <dialog className={styles.dialog}>{children}</dialog>
</>

Pass:

dialog::backdrop {
  background: var(--black-a6);
  backdrop-filter: blur(4px);
}

native-placeholder-styling

Use::placeholder for input placeholder styling, not wrapper elements.

Fail:

<div className={styles.inputWrapper}>
  {!value && <span className={styles.placeholder}>Enter text...</span>}
  <input value={value} />
</div>

Pass:

input::placeholder {
  color: var(--gray-9);
  opacity: 1;
}

native-selection-styling

Use::selection for text selection styling.

Pass:

::selection {
  background: var(--blue-a5);
  color: var(--gray-12);
}

Output Format

When reviewing files, output findings as:

file:line - [rule-id] description of issue

Example:
components/button/styles.module.css:12 - [pseudo-content-required] ::before missing content property
components/lightbox/index.tsx:45 - [transition-over-js-library] Using motion layoutId instead of View Transitions API

Summary Table

After findings, output a summary:

References

• MDN Pseudo-elements Reference
• View Transitions API