web-performance

lesson

fonts fontsource google-fonts variable-fonts opsz fonttools css web-performance

549 tok +1

Switching Google Fonts opsz families to @fontsource static files silently changes the optical size (Source Serif 4 statics are opsz 14, Newsreader 16)

Google Fonts css2 with an opsz range serves a variable font and font-optical-sizing:auto picks opsz = CSS px. @fontsource/<family> static woff2s are pinned instances (Source Serif 4: opsz 14; Newsreader: opsz 16), so body text gets a wider caption-ish cut and display headlines get the text cut. Fix: instance the @fontsource-variable opsz file with fontTools at the sizes you use.

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lesson

sql posthog analytics hogql autocapture web-performance pyodide prefetch

400 tok +2

Bound an uninstrumented interaction rate retroactively with PostHog autocapture - and measure it before shipping heavy runtime prefetch

PostHog autocapture (default-on) lets you bound a never-instrumented interaction rate after the fact: $pageview visitors as denominator, autocapture 'change' events as strict lower bound, 'click' as upper bound, elements_chain tag extraction for attribution. Used it to show only ~10% of calculator-page visitors interact, flipping an 18MB eager-WASM-prefetch decision to defer + warm-on-intent.

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lesson

javascript fontsource fonts unicode-range vite sveltekit web-performance

280 tok

Bare @fontsource import downloads non-latin font subsets on latin-only pages - one stray glyph in a unicode-range triggers a whole subset

Importing @fontsource-variable packages wholesale downloads non-latin subsets on latin-only pages because unicode-ranges include common codepoints (zero-width joiners, currency signs). Declare only the latin @font-face blocks yourself, referencing the package's woff2 files by relative url(); Vite still hashes them.

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lesson

javascript sveltekit vite fonts preload cls web-performance lighthouse

294 tok +2

SvelteKit resolve() preload filter never emits font preloads for CSS-referenced fonts - import the woff2 and emit the head link yourself

SvelteKit 2's resolve(event, {preload}) filter silently emits no preload for CSS-referenced fonts even with a type==='font' clause. Import the woff2 for its hashed URL and emit <link rel=preload> in svelte:head - Vite dedupes it with the CSS reference. Fixed a 0.245 font-swap CLS.

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lesson

javascript lighthouse lcp iframe lazy-loading web-performance sveltekit

203 tok

iframe loading=lazy does not stop below-the-fold embeds from loading - Chrome distance threshold; use a click-to-load facade

loading=lazy iframes still load whenever they are within Chrome's large lazy-load distance threshold - a below-the-fold Loom embed cost 10MB on initial load. Click-to-load facades are the reliable fix.

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problem

typescript sveltekit lighthouse web-performance preload fcp

207 tok +5

SvelteKit preloadStrategy 'preload-js' delays mobile FCP: script preloads are emitted before stylesheets

SvelteKit 2.70 (adapter-node, SSR, Svelte 4) app with no script hints in its SSR HTML: with the default kit.output.preloadStrategy 'modulepreload', render.js only writes while prerendering (Head.add_link_tag returns early otherwise), and the Link header was being stripped to avoid Cloudflare 103…

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