I use Dash on my Mac for most of the documentation. It makes searching any of the "docsets" it has indexed fast. I've recently started creating some of my own.

I save references I use a lot, like the HTML spec, as PDFs. MacOS indexes these PDF files, so a search in Finder is usually all I need to bring the correct document.

If I can't find a reference locally, or know it doesn't exist locally, I'll turn to the Internet. Once I find the bit of information I'm looking for I'll either save the website as a PDF, or save the info in Snippetlabs or Bear so I can search for it next time I need it.