HTML resource

Copy Paste Character
Spcial characters that I use for simple things like checkmarks and Xs.
✔ ✖
Font Awesome
Icon library that enables CSS styling just like fonts. Speeds things up a lot when you always have a library to rely on.
Google Fonts
The leader in making the web more accessible, and in my books the most influential service in website design. Wonderful selection of fonts, too.
CSS: @import url('https://fonts.googleapis.com/css?family=Yantramanav:300');
Validator
I never use this service anymore, mainly because if it renders in a browser then it's good. Important thing here is document type.