text-size-adjust

On mobile devices, the text-size-adjust property allows Web authors to control if and how the text-inflating algorithm is applied to the textual content of the element it is applied to.

As this property is non-standard, it must be used prefixed: -moz-text-size-adjust, -webkit-text-size-adjust, and -ms-text-size-adjust.

Browsers on smartphones don't display web pages using the same algorithms as browsers rendering web pages on desktop machines. Instead of laying out the web page at the width of the device screen, they lay it out using a viewport that is much wider than the device screen, usually of 800 or 1000 pixels wide. One of two possible methods is used to map back to the original device coordinates: either a smaller window is then used to display on the device screen only part of what is actually being rendered, or the viewport is stretched to the size of the device.

In essence, this means that either only a small, rectangular portion of the original web page is usually visible on a mobile device screen or that on mobile devices, the web page looks "zoomed out" and smaller than intended.

This approach has been used as numerous web sites are not able to cope with small screens and are not usable when using the classical layout algorithm. This technique has drawbacks: the text is so tiny that it is unreadable without zooming. Even so, the user experience is better with the classical approach, which often leads to put secondary content at the top of the smartphone display, effectively pushing the main content of the page outside the viewport.

But when the user zoomed in until the text becomes readable, an horizontal scrollbar is often needed as the line becomes larger than the width of the screen. This horizontal scrollbar has to be used to read each line of the text, which is a user experience nightmare. To prevent this, several mobile browsers implement a text inflation algorithm. When a focused element containing text use 100 % of the width of the screen, its text size is increased until it reached a readable size, without modifying the layout and removing the need of an horizontal scrollbar.

The text-size-adjust property allows Web authors to opt out from this behavior, as Web pages designed to handle small screen widths do not need it.

Syntax

/* Text is never inflated */
text-size-adjust: none;

/* Text may be inflated */
text-size-adjust: auto;

/* Text may be inflated in this exact proportion */
text-size-adjust: 80%;

/* Global values */
text-size-adjust: inherit;
text-size-adjust: initial;
text-size-adjust: unset;

Values

none

Is a keyword preventing the usage of the inflation algorithm on the element. Basically, this means that the element will be displayed without its font inflated. On old WebKit-based desktop browsers (Chrome≤26, Safari≤5), this will instead prevent the user from zooming the webpage in or out.

auto

Is a keyword allowing the usage of the inflation algorithm on the element. Browsers are not forced to use it and they won't do it on desktops and on some tablets. Its main use is to cancel a none value previously set.

<percentage>

A percentage value is a synonym of the auto keyword, with the additional information that the percentage value is the increase rate to be applied to the font size when the inflation algorithm is applied.

Formal syntax

none | auto | <percentage>

Specifications

Browser compatibility

[1] There is a bug in old Webkit-based desktop browsers. If -webkit-text-size-adjust is explicitly set to none, Webkit-based desktop browsers, like Chrome or Safari, instead of ignoring the property, will prevent the user to zoom in or out the Web page. See Bug 56543 (affected Safari≤5 & Chrome≤26), Bug 163359, and Bug 84186.

[2] If the viewport is set using <meta> element, the value of the CSS text-size-adjust property is ignored. See detailed implementation hints on MSDN.

See also

• Apple's documentation.
• Gecko's behavior description by L. David Baron.
• Microsoft's documentation.