A common requirement when undertaking interface design for Sharepoint is to make the interface accessible (accessible meaning that users with visual or other impairment).

In the UK, the Disability Discrimination Act implicitly states that an intranet should be accessible to all employees, so even your private intranet should be capable of coping with user impairments.

The Sharepoint Team issued a statement a while back (pre release) about the accesibilty capabilites of Sharepoint

Customising the Sharepoint interface to make this work can be a real uphill task. The interface uses a lot of nested tables making relative font sizing difficult as noted by Diantha. The Sharepoint accessibility kit can be your friend here though.

An example of the kits usage is show below - a resizing tool to change absolute to relative sizing

But beware, this does not 'magically' fix your accessibility problems and you still need to consider the interface requirements early on.

The kit also includes a set of more accessible style sheets that can be applied

Another great resource for Sharepoint design, including accessibility is Heather Solomon