How does the <font> size attribute compare to the CSS px?
The comparison of the size attribute and its CSS equivalent is undefined. Or browser-defined, if you like, but you might not find it documented. And it depends on the setting of the browser's font size. As the HTML 4 specification puts it, in the description of the SIZE attribute for FONT: "An integer between 1 and 7. This sets the font to some fixed size, whose rendering depends on the user agent. Not all user agents may render all seven sizes."