Audit website images and guide conversion to WebP and AVIF, formats that reduce image transfer size. Identify affected files, add fallback images, and review dimensions and loading behavior alongside the impact on initially visible content.
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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.
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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.
<picture> with <source type="image/avif"> and <source type="image/webp"> plus an <img> fallbackScan 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.
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.
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.
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
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Create and review custom 404 pages that help visitors recover from broken or moved links through search and navigation. Check the rendered page and HTTP response so missing content returns a proper 404 status.
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Audit or create a dedicated About page that explains a website’s purpose and identifies its people or organization. Review the rendered output and supporting information, including expertise and contact details, to find gaps.
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Review and add automated accessibility checks for components, pages, and continuous integration workflows. Use axe-based tools to detect structural and attribute issues, while identifying checks that still need keyboard or screen reader testing.
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Review sign-in, registration, multi-factor authentication, and account recovery for accessibility barriers. Find blocked paste, difficult code entry, and inaccessible backup methods, then suggest authentication paths that support password managers, autofill, or device approval.
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Review form errors, success messages, and live updates so screen reader users receive important notifications. Check the HTML delivered to the browser, announcement settings, display duration, and dismissal controls, then identify specific elements that need changes.
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Review HTML tables so screen reader users can understand how headers and data relate. Check header markup, table descriptions, and merged cells, then identify specific fixes and ways to verify them.
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