web-basics

A beginner-friendly guide to HTML, CSS, and how web browsers display pages. HTML provides the page structure, while CSS controls its appearance.

In plain words
What is it for?
It helps explain or edit headings, links, images, forms, buttons, layout, colors, spacing, and other webpage styles.
Why use it?
It explains the basic building blocks of a webpage so changes to markup and styling are easier to understand.

Skill for Claude CodeCodex

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

agentmods
npx agentmods add skills/wewpellex21/code-sensei/web-basics
Any agent
npx skills add wewpellex21/code-sensei --skill web-basics
Clone the repo
git clone --depth 1 https://github.com/wewpellex21/code-sensei

Made for: Claude Code, Codex.

Per session 38 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 620 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. Scan, not verified.
Origin 100% copy Near-identical to another mod in the catalogue.
Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

ModelPer sessionOnce invoked
Fable 5 $0.00038 $0.00620
Opus 5 $0.00019 $0.00310
Sonnet 5 $0.00008 $0.00124
Haiku 4.5 $0.00004 $0.00062

Measured 2d ago against content hash 9f38d2d9a64c, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

web-basics scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 2d ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

Origin

This is a copy

100% identical to web-basics — 0 lines differ, which has more behind it and is treated as the original. This page carries a canonical link to it rather than competing with it.

skills/web-basics/SKILL.md · 45 lines

How it starts

The opening of the file, as written. The whole thing — 45 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Web Basics — CodeSensei Teaching Module

HTML

  • Analogy: HTML is the skeleton of a webpage. It defines the structure — where the heading goes, where the paragraph goes, where the image goes. Like the blueprint of a house showing where rooms are.
  • Key insight: HTML uses "tags" that come in pairs: <tag>content</tag>. The tag tells the browser what KIND of content it is.
  • Key tags to teach:
    • <h1> to <h6> — headings (h1 is the biggest, like a newspaper headline)
    • <p> — paragraph (a block of text)
    • <a> — link (a door to another page)
    • <img> — image (a picture on the wall)
    • <div> — container (a box that groups things together)
    • <form> — form (like a paper form you fill out)
    • <button> — a clickable button
  • Quiz idea: "If you wanted to add a clickable link to Google, which tag would you use?"

CSS

  • Analogy: CSS is the paint, wallpaper, and furniture of the house. HTML says "there's a heading here," CSS says "make it blue, centered, and 32px big."
  • Key insight: CSS works by SELECTING an HTML element and then STYLING it. It's always: "who are you targeting?" + "what should they look like?"
  • Key concepts:
    • Selectors (which elements to style)
    • Properties (what to change: color, size, position)
    • Values (the specific setting: red, 16px, center)
    • Classes (reusable style labels, like "VIP" or "highlighted")
  • Quiz idea: "If the text is too small, which file would you change — the HTML or the CSS?"

How Browsers Work

  • Analogy: A browser is like a translator + artist. It reads your HTML (the instructions), applies your CSS (the design specs), and paints the result on screen.
  • Key insight: When someone visits your site, the browser downloads your files and assembles them in real-time. That's why you see the page "load."
  • Teaching flow: User types URL → Browser requests files from server → Server sends HTML/CSS/JS → Browser assembles and displays the page

The Relationship

HTML = Structure (what's on the page)
CSS  = Style (how it looks)
JS   = Behavior (what it does)

Think of it as: HTML is the house frame, CSS is the interior design, JavaScript is the electricity and plumbing that makes things work.

Read the full file on GitHub · 45 lines

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

  1. 2d ago First seen · 45 lines · 38 tokens per session scan A 9f38d2d9a64c

Subscribe to this mod's changes

web-basics is a skill published in the GitHub repository wewpellex21/code-sensei (3 stars, last pushed 4d ago), licensed MIT. It adds 38 tokens to every session and 620 once invoked, about $0.0002 per session on Opus 5. A static security scan graded it A with 0 findings. It is 100% identical to web-basics, differing in 0 lines, and is treated as a copy.

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