- design × 55 - a broad term for something that has been created from a plan, there are thousands of types of design types (Web Design, Graphic Design, 3D Design, Etc.)
- website-design × 275 - the process of planning and creating websites that can be read/viewed by web browsers
- web-design × 55 - a broad term used to encompass the way that content is delivered to an end-user through the World Wide Web, using a web browser or other web-enabled software to display the content.
- responsive-webdesign × 26
My two-cents would be to combine
web-design, but leave the others separate.
Personally I would favor just
responsive-webdesign since this isn't a broad term and most people associate the word responsive with website design in any case and it would be more favorable in my honest opinion to have people tagging both website-design and responsive at the same time rather than one or the other since both are related and would enhance the browsing through tags.
responsive-webdesign is a different concept than the rest. It should be left separate. We should write a tag wiki for it. It should be renamed to be consistent with the main tag for design.
I could see the case for design being separate as it could have different meanings. However in the context of this site, which is all about websites, it seems to refer to "web design" in all the tagged questions. The only exception that I notice is the question What is good URI design? which is talking about how URLs should look. When merging the tags, I would un-tag "design" from this question.
I would pick "web-design" as the canonical name for these three tags.