Um... how in word it is possible to judge about the absolute font sizes in pixels, if there is a minimum DPI of the document and scale? But if you dig deep, so probably still some major changes there. I do not know now, and during 97go a word when you change the default printer sometimes slid the placement of text on pages. WYSIWYG, damn :)
On the other hand, browser-based pt seems to depend on the default DPI of the monitor, which is installed in the OS (here can lie).
Regarding the original question, perhaps there is a bit of curve 1 output: to do a word text, 100pt, measure its size in pixels, then select in the browser the same size in pixels (thus it will not depend on the exposed DPI in the OS). When matched then the number of points in the word is numerically equal to the percentage in the browser. Ie 100pt word = 100% in the browser.