[css3-fonts] default font features

I wanted to point out a somewhat subtle point about default font
features and how implementations so far have implemented support for

Doing some tests [1] I noticed that current implementations treat
default features differently.  In both Webkit and IE10, if the
'font-feature-settings' property is 'normal' then no features are
applied.  Common ligatures, which are a default feature, are not
applied.  Language-sensitive forms are also not applied.

Consider this example:

  #p1 { font-feature-settings: normal; }
  #p2 { font-feature-settings: "blah" on; }

  <p id="p1">final AW Ta To <span lang="sr">б</span></p>
  <p id="p2">final AW Ta To <span lang="sr">б</span></p>

Since "blah" isn't a feature enabled by any font in use, these two
lines *should* render the same.  They do in Firefox but in Webkit
browsers the first line shows without kerning and ligatures, the
second line with.  IE10 doesn't enable kerning and ligatures but it
does show language specific forms in the second line (some fonts have
a Serbian form of the 'б' character, which the lang="sr" tag should

The reason for the difference in both cases is a value other than
'normal' for font-feature-settings causes the browser to switch to
some form of "typographic rendering mode" which enables a particular
set of default features.  Having different modes is done for
performance reasons to avoid the overhead of running through the
OpenType layout process for all text.

I think it's important for improving text rendering on the web that
default features be on by default.  There's been agreement on that in
the past but it's possible the implications of that for
implementations were not clear to everyone.  Rather than having two
modes for text rendering that change based on factors that aren't
clear to authors or users (e.g. the use of 'font-variant',
'font-feature-settings' or the use of a complex script such as
Arabic), text should appear the same, independent of short cuts and
optimizations.  User agents can of course use fast path codepaths but
the resulting text quality should always be the same as the
unoptimized path.

For situations that require bulk text rendering, authors can
explicitly enable fast-path rendering by disabling default features.


John Daggett

[1] Simple kerning and ligature tests

Received on Friday, 6 July 2012 06:37:48 UTC