QUOTE
5)If and how to support mixed branching I better explain it
<html><body> ... </body> ;What is ended here body or both?
and what would </html> do later in the html source.

Only <body> should be ended.
Well bad example because usually you close <body> and <html> on the same line.
However you should render like :
CODE
<b><u>Bold and underline</u>
just bold</b>


QUOTE
7)What to do with unknown tags like <Gv%7F#> ? Would I print it out
then or ignore it as I would do with <!--blaa just some comments-->

Should print out.
Imagine for instance you want to browse a math help page which echoes : "x < 14 && x > -1"
Well for this example, strict W3C compliance requires to have the source code written like :
CODE
x &lt; 14 &amp;&amp; x &gt; -1

However, much coders do not respect this compliance and use the fact that most browsers (in fact IE + mozilla + konqueror) renders WYSIWYG the code "x < 14 && x > -1"