In the file: Lib/_markupbase.py, function: "_parse_doctype_element" there is:

if '>' in rawdata[j:]:
    return rawdata.find(">", j) + 1

rawdata[j:] is being scanned twice.

It would be better to do:
pos = rawdata.find(">", j)
if pos != -1:
    return pos + 1


Same thing in the function: "_parse_doctype_attlist":

if ")" in rawdata[j:]:
    j = rawdata.find(")", j) + 1
else:
    return -1

It would be better to do:
pos = rawdata.find(")", j)
if pos != -1:
    j = pos + 1
else:
    return -1