Under Windows XP Pro using IE6.0 (or Opera7 or Mozilla) I am unable to make "font-weight: bolder" work when used with the heading tags (H1, H2 etc). "Font-weight: bolder" does work when used with the <p> tag.
Other attributes such as "color" and "font-size" work with no problems.
"font-weight: bolder" works under Win98SE with IE5.5 and IE6.0 with both the <p> and the <h1> tags.
I have verified this both on my own computer in another partition and on the office computers where there is a mix of Win98SE and WinXP-Pro computers.
I have set up two pages on my test site: one using "font-weight: bolder" and the other using "font-weight: lighter":