Why the fonts one font in "Word" and the browser differ? How to force the browser to do as the position?

Because when rendering the font is tied to the pixel grid and result in browsers and other normal programs, row width is the sum rounded to (sub)pixel widths of all the characters, that is, accumulates a rounding error, the greater, the longer the line. A program that is designed to work with documents for printing on paper and therefore comply with the concept of WYSIWYG, avoid the accumulation of errors agrogene or not tying the font to the pixel grid, or by adjusting the spacing between characters.

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 :)
\r
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).
\r
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.

This is certainly not the answer to the question but still: maybe in Opera as it should and in word no? If You need to move the text to print focus on PDF if you need it just to show people on the Internet, just drive.

The font size seems to be the same, the spacing between letters is just different, it's an additional setting which is set separately.

Let's see how it will look in the font in Adobe InDesign (I think it is on the printed layout is focused from their rulers, although it may be different) or other professional program prepress — plagued by vague doubts that not show correctly, and Word and Opera :)