Logical and physical styles

Once text is in place, it’s common to add inline styles, which can be achieved by way of
logical and physical styles. Many designers are confused by the difference between the
two, especially because equivalents (such as the logical strong and physical b) tend to be
displayed the same in browsers. The difference is that logical styles describe what the content
is, whereas physical styles merely define what the content looks like. This subtle difference
is more apparent when you take into account things like screen readers.
In the markup I like to <em>emphasize</em> things, a screen reader emphasizes the
text surrounded by the em tags. However, replace the em tags with i tags and the screen
reader won’t emphasize the word, although in a visual web browser the two pieces of
markup will almost certainly look identical.