Responsive Fonts for the Responsive Web

By - (Last Modified: January 31, 2015)

We all want to design great typographic experiences. We also want to serve users on an increasing range of devices and contexts. But today’s webfonts tie our responsive sites and applications to inflexible type that doesn’t scale. As a result, our users get poor reading experiences and longer loading times from additional font weights.

As typographers, designers, and developers, we can solve this problem. But we’ll need to work together to make webfonts more systemized and context-aware. Live webfont interpolation—the modification of a font’s design in the browser—exists today and can serve as an inroad for using truly responsive typography.

An introduction to font interpolation

Traditional font interpolation is a process used by type designers to generate new intermediary fonts from a series of master fonts. Master fonts represent key archetypal designs across different points in a font family. By using math to automatically find the in-between of these points, type designers can derive additional font variants/weights from interpolation instead of designing each one manually. We can apply the same concept to our webfonts to serve different font variants for our users. For example, the H letter (H glyph) in this proof of concept (currently for desktop browsers) has light and heavy masters in order to interpolate a new font weight.

Diagram of three H letters of thin, medium, and bold thickness with poles and axes labeled.

An interpolated H glyph using 50 percent of the light weight and 50 percent of the black weight. There can be virtually any number of poles and axes linked to combinations of properties, but in this example everything is being interpolated at once between two poles.

Normally these interpolated type designs end up being exported as separate fonts. For example, the TheSans type family contains individual font files for Extra Light, Light, Semi Light, Plain, SemiBold, Bold, Extra Bold, and Black weights generated using interpolation.

An example of all the fonts in the TheSans type family.

Individual font weights generated from interpolation from the TheSans type family.

Interpolation can alter more than just font weight. It also allows us to change the fundamental structure of a font’s glyphs. Things like serifs (or lack thereof), stroke contrast/direction, and character proportions can all be changed with the right master fonts.

Diagram of a three-dimensional cube of e letters showing how the appearance of the e can be changed gradually.

A Noordzij cube showing an interpolation space with multiple poles and axes.

Although generating fonts with standard interpolation gives us a great deal of flexibility, webfont files are still static in their browser environment. Because of this, we’ll need more to work with the web’s responsiveness.

Web typography’s medium

Type is tied to its medium. Both movable type and phototypesetting methods influenced the way that type was designed and set in their time. Today, the inherent responsiveness of the web necessitates flexible elements and relative units—both of which are used when setting type. Media queries are used to make more significant adjustments at different breakpoints.

A diagram of common responsive web design breakpoints, illustrating how the menu is hidden behind a triple-bar icon when there is no longer enough space to fit the text-based menu in the header.

An approximation of typical responsive design breakpoints.

However, fonts are treated as another resource that needs to be loaded, instead of a living, integral part of a responsive design. Changing font styles and swapping out font weights with media queries represent the same design compromises inherent in breakpoints.


Breakpoints set by media queries often reflect the best-case design tradeoffs—often during a key breakpoint, like collapsing the navigation under a menu icon. Likewise, siloed font files often reflect best-case design tradeoffs—there’s no font in between The Mix Light and The Sans SemiLight.

Enter live webfont interpolation

Live webfont interpolation just means interpolating a font on the fly inside the browser instead of being exported as a separate file resource. By doing this, our fonts themselves can respond to their context. Because type reflows and is partially independent of a responsive layout, there’s less of a need to set abrupt points of change. Fonts can adhere to bending points—not just breaking points—to adapt type to the design.

Diagram showing a thin H letter on the left and a bold one on the right. In between is an empty space illustrating how an H of any weight can be created using the thin and bold letters.

Live interpolation doesn’t have to adhere to any specific font weight or design.


With live font interpolation, we can bring the same level of finesse to our sites and applications that type designers do. Just as we take different devices into account when designing, type designers consider how type communicates and performs at small sizes, low screen resolutions, large displays, economical body copy, and everything in between. These considerations are largely dependent on the typeface’s anatomy, which requires live font interpolation to be changed in the browser. Properties like stroke weight and contrast, counter size, x-height, and character proportions all affect how users read. These properties are typically balanced across a type family. For example, the JAF Lapture family includes separate designs for text, display, subheads, and captions. Live font interpolation allows a single font to fit any specific role. The same font can be optimized for captions set at .8em, body text set at 1.2em, or H1s set at 4.8em in a light color.

Comparison of the headline “LIVE FONT INTERPOLATION” set in the text version and the display version of the JAF Lapture font, illustrating how the display version reads better visually.

JAF Lapture Display (top) and JAF Lapture Text (bottom). Set as display type at 40 pixels, rendered on Chrome 38. Note how the display version uses thinner stroke weights and more delicate features that support its sharp, authoritative character without becoming too heavy at larger sizes. (For the best examples, compare live type in your own device and browser.)
A paragraph typeset in the text version of JAF Lapture, illustrating how the text version reads better.

JAF Lapture Text. Set as body copy at 16 pixels, rendered on Chrome. Note how features like the increased character width, thicker stroke weights, and shorter ascenders and descenders make the text version more appropriate for smaller body copy set in paragraph blocks.
A paragraph typeset in the display version of JAF Lapture, illustrating how the text version reads better.

JAF Lapture Display. Set as body copy at 16 pixels, rendered on Chrome.

Live font interpolation also allows precise size-specific adjustments to be made for the different distances at which a reader can perceive type. Type can generally remove finer typographic details at sizes where they won’t be perceived by the reader—like on far-away billboards, or captions and disclaimers set at small sizes.

Adaptive Relationships

Live font interpolation’s context-awareness builds inherent flexibility into the font’s design. A font’s legibility and readability adjustments can be linked to accessibility options. People with low vision who increase the default text size or zoom the browser can get type optimized for them. Fonts can start to respond to combinations of factors like viewport size, screen resolution, ambient light, screen brightness, and viewing distance. Live font interpolation offers us the ability to extend great reading experiences to everyone, regardless of how their context changes.

Live font interpolation on the web today

While font interpolation can be done with images or canvas, these approaches don’t allow text to be selectable, accessible via screen readers, or crawlable by search engines. SVG fonts offer accessible type manipulation, but they currently miss out on the properties that make a font robust: hinting and OpenType tables with language support, ligatures, stylistic alternates, and small caps. An SVG OpenType spec exists, but still suffers from limited browser support.

Doing it responsibly

Our job is to give users the best experience possible—whether they’re viewing the design on a low-end mobile device, a laptop with high resolution, or distant digital signage. Both poorly selected and slowly loading fonts hinder the reading experience. With CSS @fontface as a baseline, fonts can be progressively enhanced
with interpolation where appropriate. Users on less capable devices and browsers are best served with standard @fontface fonts.

A call to responsive typography

As font designers, typographers, designers, and developers, the only way to take advantage of responsive typography on the web is to work together and make sure it’s done beautifully and responsibly. In addition to implementing live webfont interpolation in your projects, you can get involved in the discussion, contribute to projects like opentype.js, and let type designers know there’s a demand for interpolatable fonts.

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