These are Physical Style elements that indicate a sequence of
characters that has a horizontal line striking through the middle. The
HTML 2 specification had STRIKE as a proposed element which did not
reach the final specification, but HTML 3 revised the syntax to S. The
HTML 3.2 recommendation reverted to the earlier syntax, and HTML 4.0
now includes BOTH usages. Many browsers support one or the other, or
even both.