Let sanitize.py report errors from html5lib and exit
Add <!DOCTYPE HTML> to *.htm except about.htm
Drop empty <style> element from about.htm
Fix an editing error with </DIV> in contents
Drop </P> as required to please html5lib

After this commit, html5lib is happy, and the generated EPUB
has been verified to have no unexpected changes compared to
commit 0694d53da8f0aa4903e741b0b26bdb2ae0019298.