Lesson 9Advanced 3 min read

Advanced CSS Techniques

Pseudo-elements

::before and ::after create virtual elements inside any element — no extra HTML needed:

CSS

.badge::before {
  content: "✓ ";
  color: #22c55e;
}

.quote::before {
  content: """;
  font-size: 3rem;
  color: var(--accent);
  position: absolute;
  top: -10px;
  left: -20px;
}

content is required — without it, the pseudo-element doesn't render. Use content: "" for decorative pseudo-elements.

Decorative lines and shapes

CSS

.section-title::after {
  content: "";
  display: block;
  width: 60px;
  height: 3px;
  background: var(--accent);
  margin-top: 0.5rem;
}

The `:has()` selector

The "parent selector" CSS developers wanted for 20 years. It selects an element based on what's inside it:

CSS

/* Style a card differently if it contains an image */
.card:has(img) {
  padding: 0;
}

/* Style a label when its input is focused */
label:has(input:focus) {
  color: var(--accent);
}

/* Highlight form groups with invalid inputs */
.form-group:has(:invalid) {
  border-left: 3px solid #dc2626;
}

The `:is()` and `:where()` selectors

Reduce repetition in complex selectors:

CSS

/* Without :is() */
article h1, article h2, article h3 {
  line-height: 1.2;
}

/* With :is() */
article :is(h1, h2, h3) {
  line-height: 1.2;
}

:where() works the same but has zero specificity — useful for resets and defaults that are easy to override.

Logical properties

Instead of left, right, top, bottom, use logical properties that work in any writing direction:

CSS

.card {
  margin-inline: auto;        /* replaces margin-left + margin-right */
  padding-block: 1rem;        /* replaces padding-top + padding-bottom */
  padding-inline: 1.5rem;     /* replaces padding-left + padding-right */
  border-inline-start: 3px solid var(--accent); /* replaces border-left */
}

This makes your CSS work automatically in right-to-left (RTL) languages like Arabic or Hebrew.

Scroll snapping

Create smooth, snappy scroll behavior:

CSS

.carousel {
  display: flex;
  overflow-x: auto;
  scroll-snap-type: x mandatory;
  gap: 1rem;
}

.carousel-item {
  flex: 0 0 80%;
  scroll-snap-align: start;
}

Items snap into place when the user stops scrolling — no JavaScript carousel library needed.

Aspect ratio

Maintain consistent proportions:

CSS

.video-container {
  aspect-ratio: 16 / 9;
  width: 100%;
}

.avatar {
  aspect-ratio: 1;
  width: 48px;
  border-radius: 50%;
  object-fit: cover;
}

Backdrop filter

Apply effects to the area behind an element:

CSS

.glass-card {
  background: rgba(255, 255, 255, 0.1);
  backdrop-filter: blur(12px);
  border: 1px solid rgba(255, 255, 255, 0.2);
  border-radius: 12px;
}

This creates the popular "glassmorphism" effect.

Color-mix() and modern color functions

CSS

.button:hover {
  background: color-mix(in srgb, var(--accent), black 20%);
}

This darkens the accent color by mixing it with 20% black — great for hover states without hardcoding a second color.

Nesting (native CSS)

Modern CSS now supports nesting, similar to Sass:

CSS

.card {
  padding: 1rem;
  border: 1px solid var(--border);

  & h2 {
    font-size: 1.25rem;
    margin-bottom: 0.5rem;
  }

  &:hover {
    border-color: var(--accent);
  }

  & .card-footer {
    border-top: 1px solid var(--border);
    padding-top: 0.75rem;
  }
}

This keeps related styles grouped together and reduces repetition.

The `@layer` rule

Control the cascade order explicitly:

CSS

@layer reset, base, components, utilities;

@layer reset {
  * { margin: 0; box-sizing: border-box; }
}

@layer utilities {
  .hidden { display: none; }
  .text-center { text-align: center; }
}

Styles in later layers always beat styles in earlier layers, regardless of specificity. This eliminates many specificity wars.

Key takeaway

Modern CSS is incredibly powerful. :has() enables parent selection, scroll snapping replaces JavaScript carousels, native nesting reduces boilerplate, @layer tames the cascade, and logical properties make your CSS internationally friendly. These aren't experimental — they're supported in all modern browsers.