7.3.3 Common Markdown Extensions ↑

7.3.3.6 Strikethrough

As an extension to the original Markdown spec from John Gruber, strikethroughs are supported, in conformance with the Markdown Guide to Extended Syntax.

In order to produce a horizontal strikethrough line, place a pair of tildes before and after the content to be marked as stricken.

So the following line…

A is true. ~~B is no longer true.~~ C is also true. 

…would produce the following effect:

A is true. B is no longer true. C is also true.