Hi Peter,
For your mixed menu, some important characters are missing in menu section.
In line 44 you have a missing closing sign ">" :
<a href="today.htm" class="but" title="Today's observations">Today</a>
In line 52, your title="" is not closed too :
<a href="infos.htm" class="but" title="Infos about our installation">About</a>
The, with that it will be better!
Jacques