Underlining or marking text is something you usually do for a test. However I thought why not take advantage of this for ordinary text on the Internet. E-books, news and blogs. Here there is no problem of having both a plain black text version or one that has markings.
The idea is then to let the author mark key words or sentences that are important. Then the reader can turn on those markings optionally . To get a faster overview or to help him remember better.
A related idea would be to let the reader turn on and off the color of the in-text hyperlinks in wikipedia articles. Otherwise it would look distracting when reading.