CSS is cascading style sheets. it is the preferred method to defining the layout of a webpage.
I will also throw out a point of caution. webforms has it's merits (few and far between from my POV). However there are also a lot of things webforms does very poorly.
1. create the illusion of state where there is none.
2. has a complicated abstraction for a simple concept like html.
3. it's so coupled to itself there are very few, if any, extension points to add custom functionality.
what you encountered (setting the height) is a perfect example. webforms created a pseudo complete set of style features for the server model of a Page rather than using CSS as the preferred choice for defining layout.
if this peaks your interest any, google "problems with webforms", "pitfalls of webforms" or "webforms vs. mvc". a lot of developers have a lot of opinions about where webforms stacks up as a web development tool.
Jason Meckley
Programmer
Specialty Bakers, Inc.
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