I know that someone will advise this, but I think the best way to achieve this is with staff. Sort of:
<frameset cols="*,350px"> <frame src="a.html" /> <frame src="b.html" /> </frameset>
This will create two frames: left and right. The left one will take the whole page minus 350 pixels.
Alternatively, you can use javascript to get the width of the entire document, subtract 350 from the total, and bind a window resize event to update the width of your liquid element (the one that occupies the rest of the page). jQuery will make this pretty easy. I'm not too sure about browser compatibility.
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