Skip to content
svelte-vitals
English
English日本語
Esc
↑↓navigate↵open⌘Jpreview
On this page

performance/render-blocking-script · Render-blocking script

A head <script src> should not block parsing.

Severity: warning

What it checks

Flags a <script src> in <head> that runs as a classic script (no type, an empty type, or a JavaScript MIME type) and has neither defer nor async, whether authored in src/app.html (caught in rendered analysis) or in <svelte:head> (caught in source analysis). A head with no <script> is not checked.

Not flagged: type="module", and non-executing types such as type="importmap", type="speculationrules", or a third-party runtime like type="text/partytown". None of these run as a blocking classic script.

Why it matters

A synchronous <script src> in <head> blocks HTML parsing until it downloads and runs, delaying first paint. defer, async, or type="module" avoids the block. SvelteKit’s own scripts are already module or deferred, so this catches hand-added blocking scripts.

How to fix

Add defer (or type="module") / async:

<script src="/analytics.js" defer></script>

Mode differences

Source analysis (the CLI, the dashboard’s static baseline) composes each route’s <head> from <svelte:head> in the page and its layout chain, followed into repo-local components, plus the known meta components (svelte-meta-tags, svelte-seo) and any you declare in metaComponents. It judges only a literal value; it never examines a dynamic one. Rendered analysis (the Vite plugin’s build pass, a route you visit in the dashboard) reads the shipped <head>, where every value is literal; the build pass covers prerendered routes only. When the two disagree, trust the rendered result.

Disabling

Record existing findings in the suppressions file (npx svelte-vitals --update-suppressions), scope the rule per route or path with overrides, or turn it off:

export default {
  rules: {
    'performance/render-blocking-script': 'off'
  }
};