I have to agree. I developed a proprietary embedded web server using a streaming HTTP parser. Complying with the HTTP parsing rules is a headache to say the least. Variable amounts of whitespace; 2 variants of line terminators (\r\n or \n) with the provision that the latter SHOULD be accepted by the server and line continuations make complying with the whole specification a real pain if you only have 100 bytes to parse pieces of your request.
Maybe for a server with massive resources (I am talking about megabytes of RAM compared to kilobytes I work with) being liberal in what you accept works, but not when you are on a budget.
Every appearance of SHOULD/MAY in a spec is just begging for bugs or incompatibility. We'd be better off if those words were banned. Spec writers would be less inclined (hopefully) to come up with all sorts of arbitrary behaviour that might happen and could be maybe handled.
Should and may are spec weasel words, in specs there must (hah!) be 'MUST' and 'MUST NOT'. Otherwise a spec is just a piece of rope with a pre-tied noose.
Maybe for a server with massive resources (I am talking about megabytes of RAM compared to kilobytes I work with) being liberal in what you accept works, but not when you are on a budget.