HTML parsers may differ on whether
<input id= onchange=f(             ends in id's or onchange's value,
<a class=`foo                      ends inside a value,
<input style=font:'Arial'          needs open-quote fixup.

Per
http://www.w3.org/TR/html5/tokenization.html#attribute-value-unquoted-state
this treats the error cases in 8.2.4.40 Attribute value (unquoted) state
as fatal errors.

\> U+0022 QUOTATION MARK (")
\> U+0027 APOSTROPHE (')
\> U+003C LESS-THAN SIGN (<)
\> U+003D EQUALS SIGN (=)
\> U+0060 GRAVE ACCENT (`)
        Parse error. Treat it as per the "anything else" entry below.

and emits ErrBadHTML.

R=nigeltao
CC=golang-dev
https://golang.org/cl/5085050