There are two holdovers from the days of typewriters that negatively affect the appearance of type in documents and on the web – the practice of putting two spaces after a period, and the use of underlining.
The use of two spaces after a period is due to typewriter fonts being monospaced. In a monospaced font, every letter takes up the same amount of space as every other. Putting two spaces after a period creates extra space and helps signal the reader that the sentence has ended. In contrast, a proportional font varies the space assigned to each letter, making words easier to read because the letters are closer together and making extra space after a period unnecessary. In fact, putting two spaces after a period in a proportional font can create a gap that is visually unappealing.
With typewriters, underlining was one of the few ways to create emphasis. Computer type has italics, bold or a different point size available to create emphasis. Also, avoid double emphasis – combining bold, italics or a larger point size with underlining – as it compromises the effect of the emphasis.