I've run into this problem before. It's related to how HTML parses table and cell widths.
You're fine setting 300 as a width as long as the contents of the element can never exceed that (setting a div with a definite width inside and an overflow rule is my favorite way).
But absent a solution like the above, the minute ANY element pushes you past that width, all bets are off. The element becomes as wide as it has to to accommodate the contents.
Additional tip - encase your width values in whatever set of quotes will nest the value properly (<table width='300'
). If someone comes along and changes it to a %, it will ignore the %, otherwise.
Unfortunately, you're always going to have trouble breaking strings that do not have 'natural' breaks in IE, unless you can do something to break them up via code.