OpenType Feature Reference

Every OpenType feature has a four-letter tag, such as liga, smcp, or onum, defined by the OpenType specification. A font's own metadata decides which tags it supports and what glyphs each one swaps in; nothing about the tag itself is universal beyond its name and general intent. Some tags are conventionally on by default (like liga and kern), and some are conventionally opt-in (like smcp and swsh), but that's a design convention, not a rule the browser enforces.

On the web, most common features have a dedicated CSS property so you don't have to remember raw tags:

A ligature replaces two or more letters that collide or look awkward side by side with a single, redesigned glyph. liga covers the common, near-mandatory cases; dlig and hlig cover optional, more decorative or historical ones that a designer might not want switched on by default.

calt lets a glyph's shape depend on its neighbors: essential in connecting script and handwriting fonts, where a letter has to physically join the one before and after it, and useful in any font that wants to avoid awkward repeated shapes in a row. salt is a simpler, non-contextual cousin: a straight one-for-one swap to an alternate letterform, applied everywhere regardless of context.

Small capitals are not just scaled-down regular capitals. A well-built font draws them at their own weight and proportions so they sit comfortably next to lowercase text. smcp converts lowercase letters to small caps; c2sc does the same for letters that are already capitals, useful for acronyms inside running text.

Most text fonts ship two figure styles and two spacing modes, combinable in any pairing. lnum (lining) figures are all the same height, like capital letters: the default in most UI and data contexts. onum (oldstyle) figures rise and descend like lowercase letters, blending into running prose. tnum (tabular) gives every digit identical width so columns of numbers align; pnum (proportional) lets digit width vary for a more natural rhythm in text.

Three small, high-utility substitutions round out numeral handling. frac turns a typed sequence like 3/4 into a properly proportioned fraction. ordn raises and shrinks ordinal suffixes (1^st, 2^nd) instead of leaving them full-size. zero adds a slash or dot through the digit zero, so it can't be confused with the letter O in codes, IDs, and technical text.

sups and subs swap in purpose-drawn superscript and subscript figures, properly weighted and positioned, unlike the browser's default fallback of shrinking and nudging the regular glyph, which tends to look thin and misaligned at small sizes. A related tag, sinf ("scientific inferiors"), is used specifically for chemical formula subscripts.

swsh swaps in flourished, calligraphic alternates for selected letters, usually capitals at the start of a word, for display and invitation-style settings. Stylistic sets (ss01 through ss20) are a more general mechanism: a type designer bundles any group of alternate letterforms under one numbered tag, and documents what each number does in the font's own specimen. There's no universal meaning to "ss01" across different fonts; always check the source.

Character variants (cv01 through cv99) work like stylistic sets but at the level of a single character rather than a named group, commonly used to offer an alternate 1, a, g, or ampersand independent of everything else. Inter's full desktop release documents more than a dozen of these on its own specimen page, including an alternate figure "1" without the base serif, mapped to cv01.

Punctuation is normally centered for mixed-case text, but next to all-capital letters, that centering reads as too low. case raises and re-centers punctuation like parentheses, brackets, hyphens, and quotation marks so they sit correctly beside full capitals, which matters most in headlines, logotypes, and all-caps UI labels.

kern is the oldest and most fundamental OpenType feature: pair-specific spacing adjustments so combinations like "AV," "To," and "Wa" don't sit with visually uneven gaps. It's a GPOS feature: it repositions glyphs rather than substituting them, and it's on by default in essentially every browser and font.

locl (localized forms) is different in kind from every feature above: instead of a stylistic choice, it's a correctness rule that depends on the language tag applied to the text, not a simple on/off switch. The classic example is Turkish, where a capital "I" lowercases to a dotless "ı," not the dotted "i" every other Latin-script language expects, so a font needs a Turkish-specific substitution to render either language correctly from the same glyph set. Indic scripts depend on locl even more heavily, since it is one of the features every Brahmic script shaping engine relies on to pick the right regional glyph forms.

Every feature covered above, with its tag and the CSS that turns it on.