I'd like to strike through a comment (or part of comment) on a trello card.
How can I achieve that, markdown, HTML?
Like this
Not currently (just tried it).
See this question, one of the answers mentions possible future support.
Markdown in card comments is listed under ideas, see the Trello dev board.
UPDATE:
As per Daniel's answer: Since February 2014 strikethrough is officially supported in Trello using the following syntax: ~~strikethrough~~.
<s>Like this</s>
. The markdown equivalent is~~Like this~~
: ~~Like this~~