What does it mean to subset a webfont file, and why does subsetting usually cut its size so dramatically?

## What is actually inside a font file A `woff2` or `ttf` file is not a picture of an alphabet; it is a small binary database of tables. The `cmap` table maps Unicode codepoints to internal glyph ids. The outline table (`glyf` for TrueType outlines, `CFF ` for PostScript ones) holds the actual vector shape of every glyph. `hmtx` holds each glyph's advance width. `GSUB` and `GPOS` are the OpenType layout tables that implement kerning pairs, ligatures, small caps, tabular figures and script-specific shaping. There is also hinting bytecode, plus name and metric metadata. A well-made open-source text family may carry well over a thousand glyphs: Basic Latin, Latin Extended-A and -B for European diacritics, Greek, Cyrillic, Vietnamese, currency symbols, arrows, mathematical operators, and several stylistic sets. A marketing site in English renders a few hundred distinct characters at most — usually far fewer. ## What subsetting does Subsetting produces a **new font file** that keeps only a chosen set of glyphs. It is not compression and it is not a CSS trick: the work happens ahead of time, on the binary, in your build. The tool walks the requested codepoints, keeps their glyphs and anything they depend on (composite accented glyphs reference base glyphs, ligature substitutions reference their components), rewrites `cmap` and `hmtx`, prunes the layout tables to the features you asked to keep, and emits the result. The canonical tool is `pyftsubset`, shipped with the Python `fonttools` package. You give it a codepoint list or ranges, the layout features to preserve, and an output flavour: ```bash pyftsubset Inter.ttf \ --unicodes="U+0000-00FF,U+2018-201D,U+2013-2014,U+20AC" \ --layout-features="kern,liga" \ --flavor=woff2 \ --output-file=inter-latin.woff2 ``` Because subsetting and format conversion are separate steps conceptually, it helps to keep them separate mentally too: `woff2` compression shrinks whatever glyphs are present, subsetting decides which glyphs are present at all. You want both. ## Choosing the character set Two strategies exist. **Static**: declare the ranges the product supports — Basic Latin, the punctuation you typeset with (curly quotes, en and em dashes), the currency symbols, and any diacritics your locales need. **Content-derived**: scan the built site's rendered text and keep exactly the codepoints that appear. Tools such as `glyphhanger` automate the scan and hand the resulting codepoint list to `pyftsubset`. Content-derived subsets are the smallest and the most fragile — they are only safe for text you control, such as a fixed display headline. ## What subsetting can break - **Missing characters fall back.** Font fallback is per character, not per element: a Cyrillic name in a user comment renders from the next family in the stack (or as a `.notdef` box) while the surrounding Latin still uses your webfont. On user-generated content that looks broken. - **Layout features vanish if pruned.** Strip `GPOS`/`GSUB` too eagerly and kerning pairs and ligatures go with them; text still renders, but spacing changes subtly. - **Hinting removal** is a size win but changes rendering at small sizes on platforms that still use hinting. - **Licensing.** Some commercial licences restrict modifying or re-hosting the binary; subsetting is a modification. Check before shipping. ## Where it sits in the wider strategy Subsetting is the cheapest byte saving in font strategy because it removes data no one will ever see, with no rendering tradeoff for the characters you keep. It composes with everything else: subset first, ship as `woff2`, split the subsets across `@font-face` rules with `unicode-range` if you genuinely serve multiple scripts, and pick weights deliberately. A team that skips it often ships several hundred kilobytes of Cyrillic and Greek outlines to an audience that reads only English — bytes on the critical path, competing with the content that actually gets rendered.