How to convert to px?

Answer 1

According to The W3C:

This attribute sets the size of the font. Possible values:

• 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.
• A relative increase in font size. The value "+1" means one size larger. The value "-3" means three sizes smaller. All sizes belong to the scale of 1 to 7.

Hence, the conversion you're asking for is not possible. The browser is not required to use specific sizes with specific size attributes.

Also note that use of the font element is discouraged by W3 in favor of style sheets.