Earlier web designers used various methodology to define text size on the web pages. Heading tags such as <h1> <h2> and <font> tags were used to specify the type size.
Cascading style sheets or CSS is now preferred by web designers who retain the heading tags but the <font> tag is completely out. The CSS enabled websites conform to the W3C recommendations and is a best practice <a href=/technology.html>web design technology</a> all over. CSS introduces a more powerful and finer level of control over the type sizes. With CSS you can size type in various units of measurements as cms, inches etc.
Even while using CCS avoid absolute type size of web page elements as by fixed pixel or point, use percentages or ems instead.This applicable since all browsers are not set up to display web pages at 72 pixels or 72 dots per inch.
Another example is inconsistency in resolving pixels into dpi by various monitors in computer interfaces. A well planned strategy in web design and development process will determine correctly the right type size Vis a Vis headings, sub headings, captions and others.