compatMode == "CSS1Compat" ?
document.documentElement : document.body) &&
(eval(d.scrollTop))
);
}
The h1 element with ID "fixed" will remain at the top of the viewport even though
the content of the main document scrolls up and away. The display is a little jerky,
which exemplifies a drawback of using expressions: a considerable performance
overhead incurred by doing so. As the values are being monitored constantly you
may find that the performance of the page becomes slower, and the display isn??™t as
smooth as usual when windows are resized or redrawn.
Expressions are a powerful feature, but they do blur the distinction between
presentation and behavior. Think carefully before you use expressions, and assess
whether the behavior you want could be implemented more cleanly via a dedicated
script.
The Ultimate CSS Reference 390
Summary
Vendor-specific extensions are proprietary properties implemented by browser
vendors and were never meant for normal consumption by authors. They follow a
pre-defined format that allows them to exist within the rules of CSS and to be
protected from conflicts with existing and future properties.
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