Use when reviewing image assets, markup, and CDN or build transforms related to Serve images at the correct display size. Check encoded size, rendered size, loading strategy, and above-the-fold impact together.
インストールする前に、エージェントに与えられる指示の中身を確認できます。
A single oversized hero image can add several megabytes to a page load unnecessarily. On mobile devices with 375px screens, serving a 1600px image downloads 4x the data users need. The 'Properly size images' Lighthouse audit consistently identifies this as one of the highest-impact performance opportunities on most websites.
srcset with width descriptors and sizes with actual layout widthssizes attribute is critical—without it the browser assumes 100vw and picks the largest imageAudit images in this codebase for oversizing. Use Lighthouse 'Properly size images' audit results as the primary signal. For each <img> in the HTML: 1) Compare the image file's intrinsic width to the element's rendered CSS width (check CSS rules, container constraints). 2) Flag any image where the intrinsic width is more than 2x the rendered width at any common viewport size. 3) Check for srcset usage—images over 200px wide should have srcset variants. 4) Check sizes attribute accuracy—does it reflect the actual CSS layout?
For each oversized image: 1) Generate multiple size variants: 400w, 800w, 1200w, 1600w (use Sharp or Squoosh). 2) Add srcset listing each variant with its width descriptor. 3) Add a sizes attribute describing the image's display width at each breakpoint (e.g., sizes="(max-width: 600px) 100vw, (max-width: 1200px) 50vw, 400px"). 4) For art direction (different crops), use <picture> with media attributes. 5) For the <img> src fallback, use the medium size (800w). Show complete before/after HTML.
Explain why serving oversized images wastes bandwidth. If a browser renders an image at 400px wide but the image file is 2000px wide, the browser downloads ~25x more pixel data than it can display (bandwidth scales with pixel count, not linear dimensions). The Lighthouse 'Properly size images' audit estimates potential savings in KiB for each oversized image. The sizes attribute is the key to accuracy—without it, the browser assumes 100vw and downloads the largest srcset candidate even on narrow viewports.
Review image assets, markup, and delivery configuration related to Serve images at the correct display size. 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/responsive-size
まだレビューはありません。使ってみた感想をお寄せください。
概要と使いどころ
Use when reviewing templates, rendered HTML, or shared components related to Create a custom 404 error page. Validate the final browser-facing markup, not just the source framework abstraction.
日本語の概要は準備中です。原文の説明を表示しています。
Use when auditing metadata, crawlability, structured data, or indexability related to Create a dedicated About page. Verify the rendered HTML and HTTP response rather than relying only on source files.
日本語の概要は準備中です。原文の説明を表示しています。
Use when reviewing component libraries, page flows, or CI pipelines that need repeatable accessibility checks. Automated testing is strongest at catching structural and attribute-level issues; it does not replace keyboard, screen reader, and manual UX testing.
日本語の概要は準備中です。原文の説明を表示しています。
Use when reviewing sign-in, sign-up, MFA, CAPTCHA, recovery, and re-auth flows. Evaluate the full authentication path, including error handling and backup methods, not just the primary login form.
日本語の概要は準備中です。原文の説明を表示しています。
Use when reviewing templates, rendered HTML, or shared components related to Make notifications accessible. Validate the final browser-facing markup, not just the source framework abstraction.
日本語の概要は準備中です。原文の説明を表示しています。
Use when reviewing rendered HTML, interactive components, or design-system patterns related to Use semantic table markup for screen readers. Check native semantics first, then inspect keyboard behavior, focus flow, accessible names, and screen-reader output where relevant.
日本語の概要は準備中です。原文の説明を表示しています。