Guide · Agent skill
How to use the CSS Containment Review agent skill
Updated · written with AI from the project's own documentation and checked against our catalogue data
The css-containment skill is a set of instructions that tells an AI agent how to review your CSS and component styles for places where CSS containment could limit how much of the page the browser must recalculate. It is written for front-end developers who want a second pass on rendering performance, and for anyone learning what the contain property does. After following this guide you will know how to install the skill, how to ask for a review in plain language, and how to judge the agent's suggestions before applying them.
CSS Containment Reviewthedaviddias/Front-End-Checklist · MIT Download the skill Quick start
- 1Download the ZIP. Get the css-containment ZIP from the skill's page in the directory.
- 2Unzip the folder. Extract it so css-containment sits in ~/.claude/skills/ for personal use or .claude/skills/ inside your project.
- 3Or upload in the Claude apps. If you use the Claude apps, go to Settings → Capabilities → Skills and upload the skill.
- 4Ask a natural request. Describe the component, list or stylesheet you want reviewed and mention any breakpoints that matter.
- 5Review the suggestions. Read the proposed contain values and the flagged selectors, and ask the agent to explain what each one does.
- 6Test in the browser. Apply changes yourself and check the rendered page across breakpoints and interaction states.
What the css-containment skill makes the agent do
By default, a CSS change anywhere on the page can affect any other element's layout. CSS containment lets you declare that a part of the page is isolated, so changes inside it do not need to propagate outward. The skill makes the agent look at your stylesheets and component styles and check whether self-contained parts could be marked with contain: layout style paint, or whether repeated off-screen list items could use content-visibility: auto.
The agent explains what each value does, proposes the change, and flags selectors or breakpoints that conflict with the rule. The skill is guidance for a human developer. It is not a tool that changes your site on its own.
- Checks for components that are visually self-contained, with no overflow and no interaction with elements outside them
- Suggests contain values for those components, and content-visibility: auto for long off-screen lists
- Explains how each value limits rendering scope
- Flags exact selectors, declarations, or breakpoints that break the rule in the rendered UI
Install the css-containment skill
Download the ZIP for the skill from the skill's page in our directory. Unzip it so the css-containment folder sits in one of the following locations. For a personal setup, use ~/.claude/skills/. For a single project, use the .claude/skills/ folder inside the project.
If you use the Claude apps instead, you can upload the skill under Settings → Capabilities → Skills.
- Personal: ~/.claude/skills/css-containment
- Project: .claude/skills/css-containment inside your project folder
- Claude apps: Settings → Capabilities → Skills, then upload the skill
Files included in the skill
The skill contains two files. SKILL.md holds the main instructions, including the check, fix, explain and code review steps. references/rule.md holds the fuller write-up with code examples and the full list of contain values, which the agent can consult for details.
- SKILL.md: the core instructions and metadata
- references/rule.md: detailed rule notes and examples
How to trigger the skill with natural requests
You do not need a special command. The skill's description covers reviewing stylesheets, component styles and responsive behavior related to limiting repaint scope. Ask in ordinary language and mention the files or components you care about. Good requests name a scope, such as a folder, a component, or a long list.
The skill also asks the agent to check the rendered layout across breakpoints and interaction states before proposing a fix, so include breakpoints or states if they matter to your page.
- Review the CSS in src/components/ for places where containment could reduce repaint scope.
- Check whether the product list styles could use content-visibility: auto.
- Look at the dashboard widget styles and tell me if contain: layout style paint is safe, including at mobile breakpoints.
- Explain what contain: strict would do to my card component before I change anything.
Safety and what the skill needs
The skill lists no requirements. It does not run scripts and does not use the network, according to its listing. Because it only reviews and proposes changes, you still decide what to apply. Containment can clip content or change layout, so test each change in the browser across your breakpoints.
The skill warns that contain: strict should be used only for widgets with no overflow that do not affect the rest of the page.
- Requires: nothing listed
- Runs scripts: no
- Uses network: no
- Always review proposed selectors and test in the rendered page
Tips for getting useful results
Give the agent a bounded area to review. A single component or one stylesheet produces more precise suggestions than the whole project. Ask it to explain each value before accepting it, since the goal is to learn when containment is safe, not just to add properties.
Skip the skill for simple static sites where rendering performance is not a concern, because the change adds complexity for little gain.
- Start with one component or list rather than the full codebase
- Ask for the reason behind each contain value
- Compare the page before and after each change in the browser
What people use it for
- Reviewing a long product or search results list to see whether content-visibility: auto fits the off-screen items
- Checking whether a self-contained dashboard widget can use contain: layout style paint
- Auditing component styles to find selectors that would break if containment were added
- Learning what each contain value does before using it in a team codebase
Frequently asked questions
How do I install the css-containment skill?
Download the ZIP from the skill's page in the directory and unzip it into ~/.claude/skills/ for personal use or .claude/skills/ inside a project. In the Claude apps, you can instead upload it under Settings → Capabilities → Skills.
Does the css-containment skill require anything else?
The listing shows no requirements, and it does not run scripts or use the network. It works as written instructions that the agent follows while reviewing your code.
Is the css-containment skill free?
The skill is published under the MIT licence. This page does not list pricing, so check the directory listing for any further terms.
Will the skill change my website automatically?
No. The skill is guidance for a developer. The agent explains values and proposes changes, and you decide what to apply and test.
When should I not use CSS containment?
Avoid it when components overflow or affect elements outside them, since contain values can clip content or change layout. It is also not worth the added complexity on simple static sites where rendering speed is not a concern.
How does this compare with just adding contain everywhere?
The skill checks each component for isolation and flags conflicts, rather than adding properties blindly. Adding contain without that check can break layouts, so reviewing each selector is the safer route.
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Our verdict
Use this skill if you are a front-end developer working on pages with many independent components, long lists or dashboards, or if you want to learn when CSS containment is safe. Pick something else if your components overflow into the rest of the page or your site is simple and rendering speed does not matter.
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