Problem: Opera can do its best to interpret the code that is sent to us, to be compatible with the standards and with other browsers BUT if the code we get is different from what the others get and plain inconsistent, we don't have any opportunity to displaying the page as the webmaster intended. Not the slightest.
Take, for example https://www.fleet.ford.com/default.asp
You will notice the overlapping content at once. A quick look at the source shows the following problem..
<div ID='contentdiv' class='contentIE'>
<div ID='contentdiv' class='contentNS'>
What Opera gets is:
<div ID='contentdiv' class=''>
Either what they send IE or what they send FF would work just fine. Opera falls between two chairs because the sniffer is intelligent enough to recognise it is neither of them but has no alternative content.