f7723559_CSS_HTML_XHTML_Reference_Book__Description_and_notes
[Property/Values] font-weight either normal, bold, bolder, lighter, 100, 200, 300, 400, 500, 600, 700, 800, 900, or inherit [Description and notes] 
for applying, removing, and adjusting bold formatting initial value: normal; the numeric values are considered keywords and not integers (you can't choose 150, for example); inherited
CSS HTML XHTML Reference Book
Description and notes