fontsize is not chaging the size

[V S Rawat]

var myStrMain = "what is happening here?";
document.write("fontsize = 10pt: " + myStrMain.fontsize("10pt") +
"<p>");

I have tried with fontsize("16pt"), fontsize("18pt"), fontsize(18),
whatever fontsize is give there, it is showing the same size of that
string in ff 2.0.0.4

what am I doing wrong?

tia.
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