HTML & CSS ⚡ Recently updatedUpdated on Jun 30, 2026 — this content may still be evolving.

HTML & CSS: Revision Sheet

One-page rapid review. Read this the night before your interview.

Box Model

content → padding → border → margin | Use box-sizing: border-box

Specificity (low → high)

Element (1) < Class (10) < ID (100) < Inline (1000) | Avoid !important

Flexbox (1D layout)

Parent: display:flex, justify-content, align-items, gap, flex-wrap Child: flex: 1, align-self

Grid (2D layout)

display:grid, grid-template-columns: repeat(3, 1fr), gap, grid-template-areas

Flexbox vs Grid

• Flexbox: navbar, card row, centering, 1D
• Grid: page layout, card grid, dashboard, 2D

Positioning

• relative — offset, still in flow
• absolute — relative to positioned parent
• fixed — viewport
• sticky — scroll-based fixed

Responsive

<meta name="viewport" content="width=device-width, initial-scale=1.0">

Mobile-first: @media (min-width: 768px)

Semantic HTML

header, nav, main, article, section, footer — not div soup

Accessibility Quick Hits

• alt on images
• <label for="id"> on form inputs
• <button> for actions, <a> for navigation
• Don’t remove focus outlines
• Color contrast 4.5:1 minimum

Centering

.parent { display: flex; justify-content: center; align-items: center; }
/* OR */
.parent { display: grid; place-items: center; }

Common Display Values

• block — full width, new line
• inline — no width/height
• inline-block — width/height, flows with text
• none — removed from layout

Pseudo-classes vs Pseudo-elements

• :hover, :focus, :nth-child() — one colon
• ::before, ::after — two colons

Script Loading

• defer — after HTML parse, keeps order (preferred)
• async — when ready, no order guarantee

CSS Variables

:root { --primary: #3498db; }
.btn { background: var(--primary); }

z-index

Only works within same stacking context. New context: positioned + z-index, opacity < 1, transform.

Quick Answers

Last check: Open your project, resize to mobile, and explain your layout choices out loud.