How do I install the HTML CSS Best Practices skill?

Run npx skillmds@latest add tomevault-io/html-css-best-practices in your terminal (requires Node.js), paste this page's agent-chat prompt into Claude, Cursor, or any MCP-connected agent, or download the SKILL.md file and copy it into your agent's skills directory.

What does the HTML CSS Best Practices skill do?

Semantic HTML5, CSS custom properties, responsive design, and performance optimization for web development Use when this capability is needed. It is listed under Web & Frontend on SkillMD.

Is HTML CSS Best Practices safe to use?

This skill has not completed SkillMD's automated safety review yet. SkillMD never runs a skill's scripts for you; review the SKILL.md before installing.

Which AI agents work with HTML CSS Best Practices?

This skill is tagged as working with Claude Code, Claude.ai, OpenAI Codex. SKILL.md is an open format, so most agents that read a skills directory can load it too.

Is HTML CSS Best Practices free to use?

Yes. Installing skills from SkillMD is free, and the skill stays under its author's original license.

Who published HTML CSS Best Practices?

tomevault-io (@tomevault-io) published this skill. Their other Agent Skills are listed on their SkillMD profile.

HTML CSS Best Practices by tomevault-io

Name: HTML CSS Best Practices
Author: tomevault-io

HTML/CSS Best Practices Skill

Purpose

This skill defines HTML and CSS best practices for building accessible, performant, and maintainable web interfaces, specifically for the Hack23 Homepage and related projects.

Rules

Semantic HTML5

MUST USE:

<header> for site/page header
<nav> for navigation menus
<main> for main content (one per page)
<article> for self-contained content
<section> for thematic grouping
<aside> for tangential content
<footer> for site/page footer
<h1>-<h6> in hierarchical order (don't skip levels)
<button> for interactive actions
<a> for navigation
<figure> and <figcaption> for images with captions
<time datetime=""> for dates/times

MUST NOT:

Use <div> when semantic element exists
Use <span> for block-level content
Skip heading levels (e.g., h1 → h3)
Use <br> for spacing (use CSS)
Use tables for layout (use CSS Grid/Flexbox)

HTML Structure

MUST:

Include <!DOCTYPE html>
Set lang attribute on <html> element
Include charset: <meta charset="UTF-8">
Include viewport meta: <meta name="viewport" content="width=device-width, initial-scale=1.0">
Use descriptive <title> (unique per page)
Include meta description
Close all tags properly
Use lowercase for element names and attributes
Quote all attribute values
Validate HTML with W3C validator

Example:

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta name="description" content="Hack23 - Cybersecurity consulting with transparency">
  <title>Hack23 - Cybersecurity Consulting</title>
  <link rel="stylesheet" href="styles.css">
</head>
<body>
  <header>
    <nav>...</nav>
  </header>
  <main>
    <article>...</article>
  </main>
  <footer>...</footer>
</body>
</html>

CSS Organization

MUST:

Use single external stylesheet (styles.css)
Organize CSS logically:
1. CSS Custom Properties (variables)
2. Reset/normalization
3. Base styles (html, body, headings)
4. Layout (header, nav, main, footer)
5. Components
6. Utilities
7. Media queries
Use meaningful class names (BEM methodology recommended)
Avoid !important (use specificity correctly)
Group related properties
Use shorthand properties where appropriate
Comment complex sections

Example:

/* CSS Custom Properties */
:root {
  --primary-color: #2c3e50;
  --secondary-color: #3498db;
  --text-color: #333;
  --bg-color: #fff;
  --max-width: 1200px;
  --spacing-unit: 8px;
}

/* Base Styles */
body {
  font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
  color: var(--text-color);
  background-color: var(--bg-color);
  line-height: 1.6;
  margin: 0;
  padding: 0;
}

/* Layout */
.container {
  max-width: var(--max-width);
  margin: 0 auto;
  padding: 0 calc(var(--spacing-unit) * 2);
}

/* Components */
.button {
  display: inline-block;
  padding: calc(var(--spacing-unit) * 1.5) calc(var(--spacing-unit) * 3);
  background-color: var(--primary-color);
  color: white;
  text-decoration: none;
  border-radius: 4px;
  transition: background-color 0.3s ease;
}

.button:hover,
.button:focus {
  background-color: var(--secondary-color);
}

CSS Custom Properties (Variables)

MUST USE for:

Colors (primary, secondary, backgrounds, text)
Spacing (margins, padding multiples)
Typography (font sizes, line heights)
Breakpoints
Transitions/animations
Z-index values

Benefits:

Easy theme switching
Consistent values across site
Easier maintenance
Runtime updates possible

Responsive Design

MUST:

Design mobile-first (base styles for mobile, media queries for larger screens)
Use relative units (rem, em, %, vw, vh) over fixed px
Test at multiple breakpoints (320px, 768px, 1024px, 1440px+)
Use CSS Grid or Flexbox for layouts
Ensure touch targets are at least 44×44px
Avoid horizontal scrolling
Use max-width for readability (60-80 characters per line)

Recommended Breakpoints:

/* Mobile-first base styles */

/* Tablet */
@media (min-width: 768px) {
  /* Tablet styles */
}

/* Desktop */
@media (min-width: 1024px) {
  /* Desktop styles */
}

/* Large Desktop */
@media (min-width: 1440px) {
  /* Large desktop styles */
}

Performance Optimization

MUST:

Minimize CSS file size (remove unused styles)
Use CSS minification in production
Avoid deep selector nesting (max 3 levels)
Use efficient selectors (avoid universal *)
Load CSS in <head> (blocking is necessary)
Consider critical CSS for above-the-fold content
Use will-change sparingly for animations
Optimize images (WebP format, lazy loading)

Image Optimization:

<!-- Use WebP with fallback -->
<picture>
  <source srcset="image.webp" type="image/webp">
  <img src="image.jpg" alt="Description" loading="lazy">
</picture>

<!-- For icons, prefer inline SVG -->
<svg width="24" height="24" aria-hidden="true">
  <use xlink:href="#icon-name"></use>
</svg>

Browser Compatibility

MUST:

Test in modern browsers (Chrome, Firefox, Safari, Edge)
Use autoprefixer for vendor prefixes
Provide fallbacks for newer CSS features
Use progressive enhancement

Example with fallback:

.grid {
  display: flex; /* Fallback */
  flex-wrap: wrap;
}

@supports (display: grid) {
  .grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(250px, 1fr));
    gap: 20px;
  }
}

Examples

Example 1: Semantic HTML Page Structure

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta name="description" content="Cybersecurity consulting services with transparent security practices">
  <title>Services - Hack23 Cybersecurity Consulting</title>
  <link rel="stylesheet" href="styles.css">
</head>
<body>
  <header class="site-header">
    <div class="container">
      <a href="index.html" class="logo">
        <img src="logo.svg" alt="Hack23 Logo" width="150" height="50">
      </a>
      <nav aria-label="Main navigation">
        <ul class="nav-menu">
          <li><a href="index.html">Home</a></li>
          <li><a href="services.html" aria-current="page">Services</a></li>
          <li><a href="projects.html">Projects</a></li>
          <li><a href="blog.html">Blog</a></li>
        </ul>
      </nav>
    </div>
  </header>

  <main class="main-content">
    <div class="container">
      <article>
        <header>
          <h1>Cybersecurity Consulting Services</h1>
          <p class="lead">Transparent, effective security for modern organizations</p>
        </header>

        <section>
          <h2>Security Assessments</h2>
          <p>Comprehensive evaluation of your security posture...</p>
        </section>

        <section>
          <h2>Compliance Support</h2>
          <p>ISO 27001, GDPR, NIS2 compliance assistance...</p>
        </section>
      </article>

      <aside class="sidebar">
        <h2>Related Services</h2>
        <nav aria-label="Related services">
          <ul>
            <li><a href="penetration-testing.html">Penetration Testing</a></li>
            <li><a href="security-training.html">Security Training</a></li>
          </ul>
        </nav>
      </aside>
    </div>
  </main>

  <footer class="site-footer">
    <div class="container">
      <p>&copy; <time datetime="2025">2025</time> Hack23 AB. All rights reserved.</p>
      <nav aria-label="Footer navigation">
        <ul>
          <li><a href="privacy.html">Privacy Policy</a></li>
          <li><a href="accessibility.html">Accessibility</a></li>
        </ul>
      </nav>
    </div>
  </footer>
</body>
</html>

Example 2: CSS with Custom Properties

/* ===========================
   CSS Custom Properties
   =========================== */
:root {
  /* Colors */
  --color-primary: #2c3e50;
  --color-secondary: #3498db;
  --color-accent: #e74c3c;
  --color-text: #333;
  --color-text-light: #666;
  --color-bg: #ffffff;
  --color-bg-alt: #f8f9fa;
  --color-border: #dee2e6;

  /* Typography */
  --font-base: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
  --font-mono: 'Courier New', Courier, monospace;
  --font-size-base: 16px;
  --font-size-sm: 0.875rem;
  --font-size-lg: 1.125rem;
  --font-size-xl: 1.5rem;
  --line-height-base: 1.6;
  --line-height-heading: 1.2;

  /* Spacing */
  --spacing-xs: 4px;
  --spacing-sm: 8px;
  --spacing-md: 16px;
  --spacing-lg: 24px;
  --spacing-xl: 32px;
  --spacing-xxl: 48px;

  /* Layout */
  --max-width: 1200px;
  --max-width-narrow: 800px;
  --header-height: 80px;

  /* Transitions */
  --transition-fast: 0.15s ease;
  --transition-base: 0.3s ease;
  --transition-slow: 0.5s ease;

  /* Z-index */
  --z-dropdown: 100;
  --z-sticky: 200;
  --z-modal: 1000;
}

/* ===========================
   Reset & Base Styles
   =========================== */
* {
  box-sizing: border-box;
}

html {
  font-size: var(--font-size-base);
}

body {
  font-family: var(--font-base);
  color: var(--color-text);
  background-color: var(--color-bg);
  line-height: var(--line-height-base);
  margin: 0;
  padding: 0;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

h1, h2, h3, h4, h5, h6 {
  line-height: var(--line-height-heading);
  margin-top: 0;
  margin-bottom: var(--spacing-md);
  color: var(--color-primary);
}

h1 { font-size: 2.5rem; }
h2 { font-size: 2rem; }
h3 { font-size: 1.75rem; }
h4 { font-size: 1.5rem; }
h5 { font-size: 1.25rem; }
h6 { font-size: 1rem; }

a {
  color: var(--color-secondary);
  text-decoration: none;
  transition: color var(--transition-fast);
}

a:hover,
a:focus {
  color: var(--color-accent);
  text-decoration: underline;
}

/* ===========================
   Layout
   =========================== */
.container {
  max-width: var(--max-width);
  margin: 0 auto;
  padding: 0 var(--spacing-md);
}

.site-header {
  background-color: var(--color-primary);
  color: white;
  height: var(--header-height);
  display: flex;
  align-items: center;
  position: sticky;
  top: 0;
  z-index: var(--z-sticky);
}

.main-content {
  min-height: calc(100vh - var(--header-height) - 200px);
  padding: var(--spacing-xxl) 0;
}

.site-footer {
  background-color: var(--color-bg-alt);
  padding: var(--spacing-xl) 0;
  border-top: 1px solid var(--color-border);
}

/* ===========================
   Components
   =========================== */
.button {
  display: inline-block;
  padding: var(--spacing-sm) var(--spacing-lg);
  background-color: var(--color-primary);
  color: white;
  border: none;
  border-radius: 4px;
  font-size: var(--font-size-base);
  cursor: pointer;
  transition: background-color var(--transition-base);
}

.button:hover,
.button:focus {
  background-color: var(--color-secondary);
  text-decoration: none;
}

.card {
  background-color: var(--color-bg);
  border: 1px solid var(--color-border);
  border-radius: 8px;
  padding: var(--spacing-lg);
  box-shadow: 0 2px 4px rgba(0, 0, 0, 0.1);
  transition: box-shadow var(--transition-base);
}

.card:hover {
  box-shadow: 0 4px 8px rgba(0, 0, 0, 0.15);
}

/* ===========================
   Utilities
   =========================== */
.visually-hidden {
  position: absolute;
  width: 1px;
  height: 1px;
  padding: 0;
  margin: -1px;
  overflow: hidden;
  clip: rect(0, 0, 0, 0);
  white-space: nowrap;
  border-width: 0;
}

.text-center {
  text-align: center;
}

/* ===========================
   Responsive
   =========================== */
@media (min-width: 768px) {
  .container {
    padding: 0 var(--spacing-lg);
  }

  h1 { font-size: 3rem; }
  h2 { font-size: 2.5rem; }
}

@media (min-width: 1024px) {
  .container {
    padding: 0 var(--spacing-xl);
  }
}

Example 3: Responsive Navigation

<nav class="main-nav" aria-label="Main navigation">
  <button class="nav-toggle" aria-expanded="false" aria-controls="nav-menu">
    <span class="visually-hidden">Toggle navigation</span>
    <span class="hamburger"></span>
  </button>
  <ul id="nav-menu" class="nav-menu">
    <li><a href="index.html">Home</a></li>
    <li><a href="services.html">Services</a></li>
    <li><a href="projects.html">Projects</a></li>
    <li><a href="blog.html">Blog</a></li>
  </ul>
</nav>

.main-nav {
  position: relative;
}

.nav-toggle {
  display: block;
  background: none;
  border: none;
  cursor: pointer;
  padding: var(--spacing-sm);
}

.nav-menu {
  display: none;
  list-style: none;
  margin: 0;
  padding: 0;
}

/* Show menu when expanded */
.nav-toggle[aria-expanded="true"] + .nav-menu {
  display: block;
}

/* Desktop: horizontal menu, hide toggle */
@media (min-width: 768px) {
  .nav-toggle {
    display: none;
  }

  .nav-menu {
    display: flex;
    gap: var(--spacing-md);
  }
}

Related ISMS Policies

Secure Development Policy - Security in frontend code

Performance Budget

CSS file size: < 50KB (gzipped)
First Contentful Paint: < 1.5s
Largest Contentful Paint: < 2.5s
Cumulative Layout Shift: < 0.1

Validation & Testing

HTML Validator: https://validator.w3.org/
CSS Validator: https://jigsaw.w3.org/css-validator/
Lighthouse: Run audits in Chrome DevTools
BrowserStack: Cross-browser testing

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HTML CSS Best Practices

File contents

HTML/CSS Best Practices Skill

Purpose

Rules

Semantic HTML5

HTML Structure

CSS Organization

CSS Custom Properties (Variables)

Responsive Design

Performance Optimization

Browser Compatibility

Examples

Example 1: Semantic HTML Page Structure

Example 2: CSS with Custom Properties

Example 3: Responsive Navigation

Related ISMS Policies

Related Documentation

Performance Budget

Validation & Testing

Frequently asked questions

HTML CSS Best Practices

File contents

HTML/CSS Best Practices Skill

Purpose

Rules

Semantic HTML5

HTML Structure

CSS Organization

CSS Custom Properties (Variables)

Responsive Design

Performance Optimization

Browser Compatibility

Examples

Example 1: Semantic HTML Page Structure

Example 2: CSS with Custom Properties

Example 3: Responsive Navigation

Related ISMS Policies

Related Documentation

Performance Budget

Validation & Testing

Related skills

Frequently asked questions

How do I install the HTML CSS Best Practices skill?

What does the HTML CSS Best Practices skill do?

Is HTML CSS Best Practices safe to use?

Which AI agents work with HTML CSS Best Practices?

Is HTML CSS Best Practices free to use?

Who published HTML CSS Best Practices?