Before:
  let g:vim_markdown_strikethrough = 1
  syn off | syn on

After:
  unlet! g:vim_markdown_strikethrough

Given markdown;
a ~~b~~ c

Execute (strikethrough):
  AssertNotEqual SyntaxOf('a'), 'mkdStrike'
  AssertEqual SyntaxOf('b'), 'mkdStrike'
  AssertNotEqual SyntaxOf('c'), 'mkdStrike'

Given markdown;
# ~~h1~~
## ~~h2~~
### ~~h3~~
#### ~~h4~~
##### ~~h5~~
###### ~~h6~~

Execute (strikethrough in atx headings):
  AssertEqual SyntaxOf('h1'), 'mkdStrike'
  AssertEqual SyntaxOf('h2'), 'mkdStrike'
  AssertEqual SyntaxOf('h3'), 'mkdStrike'
  AssertEqual SyntaxOf('h4'), 'mkdStrike'
  AssertEqual SyntaxOf('h5'), 'mkdStrike'
  AssertEqual SyntaxOf('h6'), 'mkdStrike'

Given markdown;
~~h1~~
=

~~h2~~
-

Execute (strikethrough in setext headings):
  AssertEqual SyntaxOf('h1'), 'mkdStrike'
  AssertEqual SyntaxOf('h2'), 'mkdStrike'