You don't.
1) What person would not bold their heading if they could (caps lock and [URGENT] tags are bad enough).
2) There's no where to go with the style; nothing renderable anyway.. The <a> tag is wrapped by a strong element (which the DOM treats as if fontWeight was set to 700). You could make it up to 900, but none of the (atleast the core) browsers care about values past 700.
Why is it that certain members are able to make the heading bold. dclamp in this section and in other areas of VBForums I see has bold in his headings.
Something that's weird is that I saw bold in the title for this thread after the first time I looked at post #2 of this thread and then backed out of it clicking on Forum Test Area. I returned to Forum Test Area just before posting this and I noticed it was no longer bold.