Example for Checkpoint
3.4 - Use relative rather than absolute units in markup language attribute values and style sheet property values.

For example, in CSS, use 'em' or percentage lengths rather than 'pt' or 'cm', which are absolute units. If absolute units are used, validate that the rendered content is usable by checking the results on various browsers or systems. See the section on Validation and Testing for more on that subject.

To Checkpoints for Guideline 3.