Font Subsetting

Load only the glyphs a page needs, without breaking the alphabets it did not expect.

involved Front-End Performance

What it adds

Per-script font subsets with declared character ranges and a fallback order for characters no subset covers.

What your agent is told to do

1
Inventory the weights, styles, and families the app genuinely renders. Most projects ship two or three weights they never use, and dropping those is a larger win than any subsetting.
2
Split each family by writing system and declare the character ranges each file covers, so a page that renders only Latin text never fetches the Cyrillic or Greek file.
3
Choose subsets from the locales the product supports and the languages its users actually write in, not from the words currently on the page.
4
Define the fallback chain explicitly and pick system fallbacks with similar metrics, so the swap from fallback to web font does not move the text under the reader's eye.
5
Do not subset by scanning the current copy for characters in use. Translations, user-generated names, and pasted content will contain characters that scan never saw, and those characters will render as empty boxes.

Edge cases it handles

Subsets must be chosen by the language coverage the product commits to. A subset built from today's marketing copy will fail the first customer whose surname carries a diacritic outside it.
A page mixing scripts — a Japanese comment under an English article, an Arabic name in a Latin table — must fetch each needed subset independently and render both correctly, with no visible mismatch in size or baseline between them.
The same face requested under two URLs, or reachable through both a subset file and a full file, downloads twice. Confirm each face is served from one canonical source across every route.
A character no loaded subset covers must fall through to a system font that has it, not render as a blank box. Decide and verify what happens for emoji, mathematical symbols, and rare punctuation.
Decide deliberately whether text is invisible while the font loads or shown immediately in the fallback, and hold that decision consistently — an app that does both looks broken.
A variable font carrying every weight may beat several static instances or may be far larger; measure the real files rather than assuming.
Font declarations, preloads, and loading behaviour belong here; the Critical CSS Strategy brief owns what is inlined for first paint, and the two must not both inline the same font-face rules.

Definition of done

Only the weights and styles the app renders are shipped.
Each family is split by writing system with declared character ranges.
A page containing a single script fetches only that script's files.
A mixed-language page renders every script correctly, each from its own subset.
No face is downloaded twice under different URLs across the app.
Characters outside every subset fall back to a system font that renders them rather than showing empty boxes.
The fallback stack is metrically close enough that the swap causes no visible reflow.
The feature matches the existing design system.
No existing functionality is broken.

Related features

Layout Shift Guard

Bundle-Aware Component Imports

UI Performance Budget

How it works

1

Copy the link

Grab the Markdown instruction URL for this feature.
2

Give it to your AI

Paste it into Claude Code, Cursor, v0, Lovable — whatever you build with.
3

It inspects, then implements

Your agent reads your existing app first, then adds the feature to fit it.

Works with your stack

These instructions are written to adapt. They tell the agent to detect your framework, match your existing design system, and reuse what you already have — rather than assuming a particular stack.

Need it tighter than that? Customize the feature and tell it exactly what you're running.