Font sizes in a line

Greetings:
I have a query regarding font sizes in a line. I was wondering if it would be possible for me to change the size of a font in a line.
for example:
This is just an arbitrary text.
I want just the arbitrary to have a font size of 12 and bold while the others have to be font size 10 and normal , with all of them being in font-family:georgia. I am able to use <div style=”font-size:10px”> to change a font but that does not do it in the same line nor do tweaking the <h> tags. I also looking for XHTML 1.1 complaince.
Is there any way I can tweak my css or my XHTML code such that i have the liberty of changing my font size, family, weight etc. just for one or 2 words in a line or paragraph.
thanks much
deviationz