Console errors include any unhandled JavaScript exceptions, network request failures, security warnings, or deprecation notices the browser emits while loading the page. Fixing this issue means resolving the underlying problem each error reports rather than suppressing the message. Even errors that do not visibly break the page can degrade performance, leak data, or trip Core Web Vitals.

Where to make the change

• Open DevTools, Console tab: expand each entry to see the file, line, and stack trace. Right-click an entry to filter, save, or jump to the source.
• Application code: patch the offending script (see js-errors). For network failures, fix the failing endpoint or remove the stale request.
• Third-party tags: if the error originates in an analytics, ad, or widget script, update the snippet, switch to the vendor's current loader, or remove unused tags.

Common causes and how to resolve them

• JavaScript runtime errors: see the JS Error Test fix; the same approach applies.
• Failed network requests (404 or 5xx): remove the dead request or fix the endpoint. Stale tracking pixels and removed assets are common culprits.
• Mixed content warnings: the page is HTTPS but loads an HTTP resource. Update the resource URL to https://.
• CORS or CSP violations: tighten or expand the policy as appropriate. CSP errors usually require allowlisting the offending origin in the response header.
• Deprecation warnings: update the call site to use the recommended API. These do not break the page today but will when the deprecation cycle completes.

Best practices

• Treat the console as a build artifact: a clean console is a sign of a healthy page. Aim for zero errors and zero warnings on production routes.
• Use error monitoring in production: real users hit edge cases your local environment will never reproduce. A monitoring service surfaces them with frequency data.
• Defer or remove unused third-party scripts: every removed script is one fewer source of console noise.

This test fails when images are served in legacy formats such as JPEG and PNG instead of modern alternatives like WebP or AVIF. Modern formats compress significantly better at equivalent visual quality, which means smaller transfers, faster page loads, and lower data usage for visitors on metered connections. Fixing this issue means generating modern variants and serving them to browsers that support them.

Example

<picture>
  <source srcset="/images/hero.avif" type="image/avif">
  <source srcset="/images/hero.webp" type="image/webp">
  <img src="/images/hero.jpg" alt="Marathon runners at sunrise" width="1600" height="900">
</picture>

Where to make the change

• Raw HTML: use the <picture> element with multiple <source> entries so the browser picks the best supported format.
• WordPress: WordPress 6.x and later supports WebP natively. Use a media optimization plugin to convert existing JPEGs and PNGs and serve modern formats automatically.
• Shopify: Shopify's image CDN can serve WebP automatically; using the image_url filter with format parameter or the image_tag helper handles negotiation.
• Wix or Squarespace: both platforms convert and serve WebP automatically when supported by the visitor's browser.
• Headless or framework sites: components like next/image and astro:assets generate AVIF and WebP variants at build time and serve the best format per request.

Common causes and how to resolve them

• Only legacy formats published: add a build step or image CDN that generates WebP and AVIF alongside the original.
• Single-format <img> tag: wrap the image in <picture> with <source> entries for AVIF and WebP, falling back to the JPEG.
• Image CDN not negotiating format: enable automatic format conversion or pass a format=auto query parameter (depends on provider).
• SVG used where a raster image would be smaller (or vice versa): illustrations and icons should be SVG; photos should be raster in a modern format. Mixing them up wastes bytes.

Best practices

• AVIF first, WebP fallback, JPEG last: AVIF compresses best, WebP is widely supported, JPEG remains the universal fallback.
• Match quality to use case: hero images can use higher quality settings; thumbnails can drop quality without visible loss.
• Use an image CDN: services like Cloudflare Images, Cloudinary, or Imgix handle resizing, format conversion, and caching in one step.

This test fails when JavaScript or CSS resources block the browser from rendering visible content. Each render-blocking file delays the first paint, and on slower networks the cumulative impact can be severe. Fixing this issue means inlining the truly critical CSS, deferring or asynchronously loading non-essential scripts, and removing unused code.

Example

<head>
  <!-- Inline only above-the-fold critical CSS -->
  <style>/* critical styles here */</style>

  <!-- Load full stylesheet asynchronously -->
  <link rel="preload" href="/styles/main.css" as="style"
        onload="this.onload=null;this.rel='stylesheet'">

  <!-- Defer non-critical scripts -->
  <script src="/scripts/app.js" defer></script>
</head>

Where to make the change

• Application code or templates: identify the CSS and JavaScript needed for the initial render and load only those synchronously. Defer the rest.
• Build pipeline: use a critical CSS extractor (such as critical or penthouse) to generate the above-the-fold styles automatically.
• WordPress: performance plugins offer "delay JS execution" and "load CSS asynchronously" options that handle most of this without code changes.
• Headless or framework sites: use the framework's preload and code-splitting hints to control what blocks rendering.

Common causes and how to resolve them

• Single large stylesheet blocking render: inline critical CSS for the first viewport, load the full stylesheet asynchronously.
• Synchronous third-party scripts in the head: add defer or async, or move them to the end of the body.
• Web fonts blocking text rendering: add font-display: swap in @font-face rules so the fallback font shows immediately.
• Render-blocking @import in CSS: replace with separate <link rel="stylesheet"> tags so the browser can fetch them in parallel.

Best practices

• Use defer for scripts that need the DOM: they execute after parsing but before DOMContentLoaded.
• Use async for fully independent scripts: they execute as soon as they download, in any order.
• Preload the critical request chain: tell the browser early about fonts and key assets with <link rel="preload">.
• Audit with Lighthouse: the Render-blocking resources audit lists every offending file with the estimated time saved by deferring it.

The ads.txt file is a public list of the advertising vendors authorized to sell your inventory programmatically. A correctly formatted ads.txt protects against domain spoofing in the ad supply chain and is required for inclusion in many premium advertising marketplaces. Fixing this issue means publishing a valid ads.txt at the site root with current authorized seller entries.

Example

google.com, pub-0000000000000000, DIRECT, f08c47fec0942fa0
example-ssp.com, 12345, RESELLER
contact=ads@example.com

Where to make the change

• Raw HTML or static site: upload ads.txt to the site root so it is served at https://example.com/ads.txt.
• WordPress: use a small plugin or upload the file via FTP. Some advertising plugins manage it automatically based on connected accounts.
• Shopify: upload the file through the theme code editor as an asset, then expose it at the root via a route configuration.
• Cloudflare or similar CDN: serve ads.txt from a worker if you do not have file-system access at the origin.

Common causes and how to resolve them

• File missing: create one with at least your direct advertising relationships.
• File at the wrong path: ads.txt must be at the domain root, not in a subdirectory.
• Wrong relationship value: use DIRECT for direct seller relationships, RESELLER for authorized resellers. Other values are invalid.
• Stale entries: partners change; remove relationships you no longer have so spoofers cannot exploit them.
• Returning HTML instead of plain text: the file must be served as text/plain. A page that returns HTML or a 404 fails validation.

Best practices

• Audit quarterly: stale entries are the main source of ads.txt vulnerabilities.
• Mirror app-ads.txt for mobile inventory: apps use a parallel app-ads.txt file with the same syntax.
• Add a contact line: include a contact=email@example.com line so vendors can reach the right person about ad-quality issues.