@crutledge - that file doesn't load for me on Sigil 0.5.3 either, or indeed on various intermediate stages of builds of 0.5.9, so I'm not convinced this is something new? You implied that it did work previously in some Sigil version, can you retry with that specific file and confirm that?
Also how did that html get generated - is it something that got manually edited, produced by some other tool or ??? I don't know enough about the html spec to know about that combination of headers as to what is valid or not. Certainly Sigil produces a subtly different html header when imported without those lines in it (different xhtml version/html namespace attributes), which it is able to handle import ok.
Just wanting to make sure we haven't introduced a regression. Certainly it would be nicer if Sigil didn't just quit after displaying its failure to parse a particular html file. But that is one of the few known issues we haven't tried to tackle - yet anyway.