This is a C-d: {bad character} right there.
Here is it wrapped up in code: {bad character} and plain {bad character} <-- there.
This may or may not validated.
No, it's not. To quote the validator on excluded characters:
(0 to 31 inclusive and 127 to 159 inclusive)
(but we want to skip tab & newline.) Steal badchars from the comments module I think.
See http://www.w3.org/MarkUp/html3/latin1.html and http://ppewww.ph.gla.ac.uk/%7Eflavell/charset/
Also in a <pre> block, for completeness:
This should be C-d right here: {bad character} <-- there.
Multiple in a <pre>: {bad character}{bad character}{bad character} .
And this is done.
Okay, this is a test of several: {bad characters} done.