publishing - Least possible editing effort if a text is for multiple media?

I want to write a text with some basic styles and images. I plan to use it on a blog, maybe later in a wiki. And there should be a pdf version.

I am writing about a software. So there will be screenshots, code blocks and some inline styling.

I want to have the least possible styling/markup effort. Any tips or good tools?

I guess what I am looking for is a tool that does the style converting for me so I don't have to do the job.