Caitlin -
1) One thing you can do to ensure your landing pages is check it against a website validator here is a link to one:
The W3C Markup Validation Service
All you do is put your web site landing page address in there and let it evaluate it.
I was curious and ran your landing page on it, and 92 errors were produced, you would have to know a little bit about HTML to fix these, but probably not too hard to repair I think.
2) Another thing you can do is make sure the CSS - the style sheet file, is standard to work on all browsers, I think another poster was attempting to tell you something on that line
hope this helps