[Theme: TwentyEleven] Header options not working in Internet Explorer

I thought everything was fine with this site until I tried it in Internet Explorer where it looks dreadful. There are a ton of problems and I hardly know where to begin (it’s my first site).

I thought I’d start with something easy, but even with that I’m stumped.

The site uses the header option not to show text with the header image (Show header text with your image is unchecked – I’ve put the text into the images themselves to save space. However this option is ignored in IE 9 and below. So the text is appearing both in the picture and the heading text.

It’s extremely difficult to troubleshoot as I am on Mac.

Everything I read says to start with validation, but that throws up a ton of errors I can’t interpret but which seem to have little to do with anything I have done.

This is just one basic error to start with so I have no idea how long the whole thing is going to take.

The site is http://www.vacationrentalscartagena.com. I now have a plugin popup for IE9 and below, but if you click the X you can continue.