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Agent skill

Modern Format

by thedaviddias in thedaviddias/Front-End-Checklist

A skill your agent uses when reviewing image assets, markup, and CDN or build transforms related to Use modern image formats (WebP, AVIF).

MITAuto-check passed

Install Modern Format

skills CLI
$ npx skills add thedaviddias/Front-End-Checklist --skill modern-format -a claude-code

Project install by default; add -g for ~/.claude/skills/.

GitHub CLI
$ gh skill install thedaviddias/Front-End-Checklist modern-format --agent claude-code

Project scope by default; add --scope user for a personal install. Needs GitHub CLI 2.90.0 or later (public preview).

Manual copy
$ git clone --depth 1 https://github.com/thedaviddias/Front-End-Checklist.git skills-src && mkdir -p .claude/skills && cp -r skills-src/skills/modern-format .claude/skills/modern-format && rm -rf skills-src

Use ~/.claude/skills/ instead of .claude/skills for a personal install. The folder must contain SKILL.md.

Claude Code skills documentation · loads skills from .claude/skills/

Facts

Skill name
modern-format
GitHub stars
74k
Token cost
~780 tokens
SKILL.md length
396 words
Files
2 (incl. references)
Skills in repo
390
Repo updated
First seen
Licence
MIT

At a glance

A skill your agent uses when reviewing image assets, markup, and CDN or build transforms related to Use modern image formats (WebP, AVIF).

  • Reviewing image assets
  • SKILL.md covers Quick Reference, Check, Fix and Explain, plus 1 more section
  • Instructions only: no scripts, shell commands, URLs or credentials in SKILL.md
  • Build transforms related to Use modern image formats (WebP

What it does

Modern Format is an agent skill from thedaviddias/Front-End-Checklist. Use when reviewing image assets, markup, and CDN or build transforms related to Use modern image formats (WebP, AVIF). Check encoded size, rendered size, loading strategy, and above-the-fold impact together.

Its SKILL.md is about 780 tokens, which your agent loads only when the skill is triggered. The skill folder holds 2 other files, including reference files (for example `references/rule.md`).

The repository describes itself as: 🗂 The essential checklist for modern web development, for humans and AI agents. The licence is MIT.

When your agent uses it

  • Reviewing image assets
  • Build transforms related to Use modern image formats (WebP

Example prompts

  • “/modern-format”

What it can do on your machine

Read from SKILL.md and the folder at commit e8d14d0. It shows what the files ask for, not the result of running them.

  • Tool permissions

    Pre-approves nothing: there is no allowed-tools line, so your agent's usual permission prompts apply.

    From allowed-tools in the SKILL.md frontmatter.

  • Runs code

    No scripts in the folder and no shell commands in SKILL.md.

    From the folder's file list and the shell code blocks in SKILL.md.

  • Network

    Links to these hosts (documentation or services it may open):

    • frontendchecklist.io

    From URLs in SKILL.md, links to its own repository left out.

  • Credentials

    Names no API keys, tokens, secrets or passwords.

    From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.

Context cost

Modern Format loads about 780 tokens when it runs, and up to ~2.1k if it reads all its reference files. Until then it costs about 55 tokens; SKILL.md has 396 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it
~55
When it runs · the whole SKILL.md, loaded when a task matches
~780
With references · SKILL.md plus every file in references/, read only if the agent opens them
~2.1k

Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.

Safety

Auto-check passed

The automated check found no risky patterns in SKILL.md.

Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); files beside SKILL.md are not scanned.

SKILL.md

The full file from thedaviddias/Front-End-Checklist at commit e8d14d0, republished under its MIT licence (© thedaviddias). 396 words, ~780 tokens.

Download SKILL.mdSave it as .claude/skills/modern-format/SKILL.md (or your agent's skills folder). This skill also uses 1 other file; get the full folder from GitHub.
name
modern-format
description
Use when reviewing image assets, markup, and CDN or build transforms related to Use modern image formats (WebP, AVIF). Check encoded size, rendered size, loading strategy, and above-the-fold impact together.
metadata.category
images
metadata.priority
high
metadata.difficulty
intermediate
metadata.estimatedTime
20
metadata.source
frontendchecklist.io
metadata.url
https://frontendchecklist.io/rules/images/modern-format

Use modern image formats (WebP, AVIF)

Images are typically 50%+ of page weight. Switching from JPEG to WebP alone reduces transfer sizes by 25-35% with no visible quality change, directly improving Largest Contentful Paint (LCP) and saving bandwidth for users on mobile data plans. These savings compound across every visitor and every image on the site.

Quick Reference

  • WebP is ~25-35% smaller than JPEG at equivalent quality (per web.dev)
  • AVIF is ~40-50% smaller than JPEG at equivalent quality (per web.dev)
  • Use <picture> with <source type="image/avif"> and <source type="image/webp"> plus an <img> fallback
  • WebP has 97%+ global browser support; AVIF is supported in all modern browsers as of 2023

Check

Scan all <img> elements and CSS background-image declarations in this codebase. Identify: 1) Any <img src> pointing to .jpg, .jpeg, or .png files (not served via a CDN that auto-negotiates format). 2) Any <picture> elements that lack <source type="image/webp"> or <source type="image/avif"> entries. 3) Any CSS background images using JPEG or PNG when a WebP equivalent could be used. Report each finding with file path and line number.

Fix

For each image not using modern formats: 1) Convert to WebP at 80% quality using Squoosh or Sharp. 2) Optionally also generate AVIF at 60% quality for browsers that support it. 3) Wrap the original <img> in a <picture> element and add <source type="image/avif"> and <source type="image/webp"> entries with the new files. 4) Keep the original JPEG/PNG as the <img src> fallback. For CSS background images, use a @supports (background-image: url('.webp')) { } query to serve WebP to supporting browsers.

Show full SKILL.md (140 more words)Show less

Explain

Explain the compression advantages of WebP and AVIF over JPEG and PNG. According to web.dev, WebP lossless images are 26% smaller than PNG and WebP lossy images are 25-35% smaller than JPEG at equivalent visual quality. AVIF offers further improvements: roughly 40-50% smaller than equivalent JPEG. Explain browser support: WebP has 97%+ global browser support as of 2024. AVIF is supported in Chrome 85+, Firefox 93+, and Safari 16.4+. The <picture> element with fallbacks means there is no risk in adopting these formats today.

Code Review

Review image assets, markup, and delivery configuration related to Use modern image formats (WebP, AVIF). Flag exact files or components where format choice, sizing, or loading behavior violates the rule, and describe how to confirm the fix in DevTools.

For full implementation details, code examples, and framework-specific guidance, see references/rule.md.

Rule page: https://frontendchecklist.io/rules/images/modern-format

© thedaviddias, MIT. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file

Files

SKILL.md and 1 other file (references) in skills/modern-format of thedaviddias/Front-End-Checklist.

  • SKILL.md
  • references/rule.md

Open the folder on GitHubat commit e8d14d0

Compare with similar skills

Modern Format next to the 5 skills that share the most tags, products or categories with it. Stars are the repository's; “used in” counts other GitHub owners with a copy.

Modern Format compared with similar skills
SkillStarsUsed inTokensAuto-checkLicenceRepo updated
Modern Format this skillthedaviddias/Front-End-Checklist74k—~780Automated safety check: PassMIT
Schema Markupalirezarezvani/claude-skills28k1 repos~2.9kAutomated safety check: PassMIT
Resume Modernnexu-io/open-design100k—~398Automated safety check: PassApache-2.0
React Modernizationwshobson/agents40k11 repos~1.7kAutomated safety check: PassMIT
Doc And Modernizegithub/awesome-copilot40k—~12kAutomated safety check: PassMIT
Schema Markup Generatorsickn33/agentic-awesome-skills47k1 repos~2.1kAutomated safety check: PassMIT

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Questions about Modern Format

What does Modern Format do?

A skill your agent uses when reviewing image assets, markup, and CDN or build transforms related to Use modern image formats (WebP, AVIF). Modern Format is an agent skill from thedaviddias/Front-End-Checklist. Use when reviewing image assets, markup, and CDN or build transforms related to Use modern image formats (WebP, AVIF).

When should I use Modern Format?

Modern Format fits situations like: reviewing image assets; build transforms related to Use modern image formats (WebP.

How do I install Modern Format in Claude Code?

Run `npx skills add thedaviddias/Front-End-Checklist --skill modern-format -a claude-code`. Or copy the skill folder (skills/modern-format in thedaviddias/Front-End-Checklist) into .claude/skills/modern-format in your project. Claude Code loads it when a task matches its description.

How do I install Modern Format in Codex?

Run `npx skills add thedaviddias/Front-End-Checklist --skill modern-format -a codex`. Or copy the skill folder (skills/modern-format in thedaviddias/Front-End-Checklist) into .agents/skills/modern-format in your project. Codex loads it when a task matches its description.

Can I use Modern Format in Cursor, Gemini CLI or GitHub Copilot?

Cursor, Gemini CLI, GitHub Copilot and OpenCode also load SKILL.md folders. With the skills CLI, run `npx skills add thedaviddias/Front-End-Checklist --skill modern-format -a cursor` (or -a gemini-cli, github-copilot or opencode for the others). To copy it by hand, put the folder in .cursor/skills/modern-format, .gemini/skills/modern-format, .github/skills/modern-format and .opencode/skills/modern-format in your project.

What does Modern Format need to run?

SKILL.md names no scripts, command-line tools or credentials: Modern Format is instructions for the agent only.

Does Modern Format access the network?

SKILL.md names 1 domain. As links in the text: frontendchecklist.io. This is read from the text; nothing was executed.

Is Modern Format safe to install?

Our automated static check of SKILL.md found no risky patterns, such as piping downloads into a shell, reading credential files or hidden Unicode. It is not a guarantee. Review the folder before installing.

What licence does Modern Format use?

Modern Format is published under the MIT licence (the repository's licence). It allows redistribution, so the full SKILL.md is shown on this page.

How many tokens does Modern Format use?

About 780 tokens (SKILL.md is roughly 3.1k characters). Agents keep only the skill's name and description in context until a task matches; then they load SKILL.md in full. Its references folder adds about 1.4k tokens, read only when the agent opens those files.

What are the alternatives to Modern Format?

Skills that share tags, products or a category with Modern Format: Schema Markup (alirezarezvani/claude-skills, 28k stars), Resume Modern (nexu-io/open-design, 100k stars), React Modernization (wshobson/agents, 40k stars) and Doc And Modernize (github/awesome-copilot, 40k stars). The comparison table on this page puts their stars, adoption, token cost, safety result and licence side by side.

Who maintains Modern Format?

thedaviddias (a GitHub user) maintains it in thedaviddias/Front-End-Checklist, which has 74,421 GitHub stars. The repository holds 390 skills in this directory. The repository was last updated on October 6, 2026.

Source: thedaviddias/Front-End-Checklist on GitHub. Facts on this page come from the repository at the commit we read; the author's words are quoted as theirs.