Non-functional requirements (NFRs) are the poor, ugly sisters to the functional requirements.  They are often left out, or worse written in wooly and non SMART terms; “the website shall be available 24/7″.  Is this what happened with HMRC?   The website that allows UK citizens to complete their tax returns on-line has gone down, just as the deadline looms.  I wonder if this is a case of the non-functional requirements around performance, scalability volumes etc being forgotten about or just not tested for.  Inexcusable really.