Google’s Paul Lewis answers the same questions that Matt Gaunt received (and that I previously linked to). Paul’s focus on the user and their experience of our work resonates strongly with me and is something I harp on quite frequently.
I think performance, accessibility, and security share some traits: they can’t be retro-fitted to a project, they’re often thankless tasks, and they’re only notable by their absence. They’re all, however, the bedrock of a good user experience, onto which you can layer high quality designs and interactions.
Paul also cites one of my favorite documents, the W3C’s HTML Design Principles:
In case of conflict, consider users over authors over implementors over specifiers over theoretical purity.