Authors should be aware that Microsoft Internet Explorer 3.x incorrectly treats em and ex units as pixels, which can easily make text using these units unreadable. The browser also incorrectly applies percentage values relative to its default font size for the selector, rather than relative to the parent element's font size. This makes rules like
H1 { font-size: 200% }
dangerous in that the size will be twice IE's default font size for level-one headings, rather than twice the parent element's font size. In this case, BODY would most likely be the parent element, and it would likely define a medium font size, whereas the default level-one heading font size imposed by IE would probably be considered xx-large.
Given these bugs, authors should take care in using percentage values for font-size, and should probably avoid em and ex units for this property.
