This is sometimes the trickiest thing to work out if you don't have any documentation that came with the fonts (many type designers and foundries will provide sample pages and CSS just for this very reason). But there are some sites that make it easier to figure out. You can visit wakamaifondue.com, drop your font file on the circle where instructed, and in a few moments you'll have a full report on all the capabilities and features of your font. Axis-praxis.org also offers a similar capability, with the ability to click on the features to turn them on or off in a given text block.

Given that these features take a bit of work to discover and use, it may seem a fair question to ask why one would bother to use them. The answer lies in the specific features that will make a site more useful, readable, and polished:

• Ligatures like 'ff' or 'fi' make letter spacing and reading more even and smooth.
• Fractions can make home improvement and recipe sites much easier to read and understand.
• Numerals within paragraphs of text set as 'oldstyle' sit more comfortably between lower-case letters, and likewise setting them as 'tabular numbers' will make them line up better when setting a list of costs in a table say. 'lining' figures on the other hand sit more uniformly on their own or in front of capitalized words.

While none of these features individually will render a site useless due to their absence, each of them in turn can make a site easier to use and more memorable for its attention to detail.

OpenType features are like secret compartments in fonts. Unlock them and you'll find ways to make fonts look and behave differently in subtle and dramatic ways. Not all OpenType features are appropriate to use all of the time, but some features are critical for great typography. -- Tim Brown, Head of Typography at Adobe.

There are some cases — like with font-variant-east-asian — that OpenType features are directly tied to using different forms of certain glyphs, which can impact meaning and readability. In cases such as these, it's more than just a nicety, but rather an integral part of the content itself.

There are a number of different features to consider. They are grouped and explained here according to the main attributes and options covered in the W3C specifications.

Associated CSS property: font-kerning

This refers to the spacing between specific glyph pairings. This is generally on by default (as recommended by the OpenType specification). It should be noted that if letter-spacing is also set on your text, that is applied after kerning.

Associated CSS property: font-variant-alternates

Fonts can supply a number of different alternatives for various glyphs, such as different styles of lower case 'a' or more or less elaborate swashes in a script typeface. This property can activate an entire set of alternates or just a specific one, depending on the values supplied. The example below is showing several different aspects of working with alternate characters. Fonts with alternate glyphs can make them available across the board or individually in separate stylistic sets, or even individual characters. In this example you can see two different typefaces, and the introduction of the @font-feature-values at-rule. This is used to define shortcuts or named options that can be defined per font family. This way you can create a named option that applies to only a single font, or one that is shared and can be applied more generally

In this case, @stylistic(alternates) will show all the alternate characters for either font). Applying this to just the word 'My' alters the way the 'M' renders, and applying @styleset(alt-a) only changes the lower case 'a'.

Try changing the line

font-variant-alternates: styleset(alt-a);

to

font-variant-alternates: styleset(alt-g);

and notice that the lower case 'a' reverts to its regular form and the lower case 'g's changes instead.

More about alternates

• https://www.w3.org/TR/css-fonts-4/#propdef-font-variant-alternates
• https://developer.mozilla.org/en-US/docs/Web/CSS/font-variant-alternates

Associated CSS property: font-variant-ligatures

Ligatures are glyphs that replace two or more separate glyphs in order to represent them more smoothly (from a spacing or aesthetic perspective). Some of the most common are letters like 'fi', 'fl', or 'ffl' — but there are many other possibilities. There are the most frequent ones (referred to as common ligatures), and there are also more specialized categories like 'discretionary ligatures', 'historical ligatures', and 'contextual alternates'. While these last ones are not technically ligatures, they are generally similar in that they replace specific combinations of letters when they appear together.

While more common in script typefaces, in the below example they are used to create arrows:

Associated CSS property: font-variant-position

Position variants are used to enable typographic superscript and subscript glyphs. These are designed to work with the surrounding text without altering the baseline or line spacing. This is especially useful with the <sub> or <sup> elements.

Associated CSS property: font-variant-caps

One of the more common use cases for OpenType features is proper small caps. These are capital letters sized to fit better amongst lower case letters and are generally used for acronyms and abbreviations.

Associated CSS property: font-variant-numeric

There are several different styles of numerals commonly included in fonts:

• 'Lining' figures are all the same height and on the same baseline.
• 'Oldstyle' figures are mixed height and designed to have the appearance of ascenders and descenders like other lower case letters. These are designed to be used inline with a copy so the numerals visually blend with the surrounding glyphs in a similar fashion to small caps.

There is also the notion of spacing. Proportional spacing is the normal setting, whereas tabular spacing lines up numerals evenly regardless of character width, making it more appropriate for lining up tables of numbers in financial tables.

There are two types of fractions supported through this property:

• Diagonal slashed fractions.
• Vertically stacked fractions.

Ordinals are also supported (such as '1st' or '3rd'), as is a slashed zero if present in the font.

Lining and old-style figures

Fractions, ordinals, and slashed zero

Associated CSS property: font-variant-east-asian

This allows access to various alternate forms of glyphs within a font. The example below shows a string of glyphs with only the OpenType set 'jis78' enabled. Uncheck the box below and you'll see more characters displayed.

The font-variant property is the shorthand syntax for defining all of the above. Setting a value of normal resets all properties to their initial value. Setting a value of none sets font-variant-ligatures to none and all other properties to their initial value. (Meaning that if kerning is on by default, it will still be on even with a value of none being supplied here.)

font-feature-settings is the 'low level syntax' that allows explicit access to every named available OpenType feature. This gives a lot of control but has some disadvantages in how it impacts inheritance and — as mentioned above — if you wish to change one setting, you have to redeclare the entire string (unless you're using CSS custom properties to set the values). Because of this, it's best to use the standard properties shown above wherever possible.

There are a huge number of possible features. You can see examples of a number of them above, and there are several resources available for finding more of them.

The general syntax looks like this:

.small-caps {
  font-feature-settings: "smcp", "c2sc";
}

According to the specification you can either supply just the 4-character feature code or supply a 1 following the code (for enabling that feature) or a 0 (zero) to disable it. This is helpful if you have a feature like ligatures enabled by default but you would like to turn them off like so:

.no-ligatures {
  font-feature-settings: "liga" 0, "dlig" 0;
}

• 'The Complete CSS Demo for OpenType Features' (can't vouch for the truth of the name, but it's pretty big)
• A list of OpenType features on Wikipedia

Since not all properties are evenly implemented, it's good practice to set up your CSS using feature detection to utilize the correct properties, with font-feature-settings as the fallback.

For example, small caps can be set several ways, but if you want to ensure that no matter what the underlying capitalization is that you end up with everything in small caps, it requires 2 settings with font-feature-settings versus a single property value using font-variant-caps.

.small-caps {
  font-feature-settings: "smcp", "c2sc";
}

@supports (font-variant-caps: all-small-caps) {
  .small-caps {
    font-feature-settings: normal;
    font-variant-caps: all-small-caps;
  }
}

• The Complete CSS Demo for OpenType Features

• Wakamai Fondue
• Axis Praxis

• Font Feature Properties in CSS Fonts Module Level 3
• font-variant-alternatives in CSS Fonts Module Level 4

• Using OpenType features by Tim Brown, Head of Typography, Adobe
• Adobe's Syntax for OpenType features in CSS