HTML and CSS Reference
to load them for all sites they visit or to customize specific sites they
The teams building web browsers have the unenviable position of need-
ing to support the way things have historically worked in their browsers
while at the same time fixing bugs and supporting new work by the
W3C. It may seem like web developers should abandon old and buggy
behavior in favor of progress, but the reality is that hundreds of thou-
sands of web sites may have been built, tested, and published with old
behaviors in mind.
As a compromise, browser vendors have come up with a way they can
be backward compatible while adhering to emerging web standards at
the same time. The solution is to support multiple rendering modes and
provide a way to switch between those modes at the document level via
the DOCTYPE declaration.
Under Standards Mode, browser rendering engines behave according to
the letter of the standards. The CSS specifications are written to be back-
ward compatible, so pages built to today's standards should not behave
differently under some new specification 10 years from now.
In other words, yet-unwritten standards will not change how color works
or how font-family designations are written even if they add features
such as font embedding or new color keywords. In extreme cases—such
as with the box model updates—a new property is created to designate
new behaviors should be followed, but the default behaviors should
match the old specifications.