Heres a quick comparison of the <font> tag with a 'size' attribute and a <span> tag with an inline style. This shows what size in pixels the 'size' attribute actually relates to. This only works if the browser text settings are set to display text at medium size and have a DPI of 96, I'm not going to go into the benefits of EM over PX in this post - this table is what it is. Especially useful when attempting to create a HTML email that correctly renders in Outlook and GMail.