Need way to "scale" text

Is there a way to create a style that “scales” whatever font it is applied to by the same percentage?

For example, lets say I have three separate lines of text…

The first sentence has a Font-Size of 10px
The second sentence has a Font-Size of 16px
The third sentence has a Font-Size of 28px

I want a style that when applied to a <span> with each sentence above, would make the selected text 50% larger than the surrounding text.

So the selected text in the first sentence would increase to 15px.
The selected text in the second sentence would increase to 24px.
And the selected text in the third sentence would increase to 42px.

If my style looked like this…

	font-size: 1.5em;

then doesn’t that just making the font 1 1/2 times the size of the base font defined in your browser?

My goal is to have a style that I can use to capitalize and increase in size to emphasize them in a sentence, however, I want the increase to be relative to the surrounding font and not some set increase.

Hope that makes sense?