How to make website look correct in Safari, Firefox, and Chrome?

Question

Hello there. I'm working on a website which was inherited to me by someone who died. So I am left with all of his coding which is about 4 years old. Now I'm working on positioning of the website and i've found that the only browser the website looks like it's supposed to is Safari and I can't for the life of me figure out how to make it look right in all the browsers and on a phone or tablet. It might be easier if I started from scratch but unfortunately I don't get that luxury. Please help me.
Here's a link...MuseumOfNaturalistory.com

Answer 1

I dont understand what some of these errors are asking of me. For instance, this one...
if (xmlhttp.readyState==4 && xmlhttp.status==200)
✉
This message may appear in several cases:
You tried to include the "<" character in your page: you should escape it as "&lt;"
You used an unescaped ampersand "&": this may be valid in some contexts, but it is recommended to use "&amp;", which is always safe.
Another possibility is that you forgot to close quotes in a previous tag.
That's the way the code has to be in order to function. What does it want from me?
And then here....
Line 71, Column 5: an attribute value must be a literal unless it contains only name characters   </div>✉You have used a character that is not considered a "name character" in an attribute value. Which characters are considered "name characters" varies between the different document types, but a good rule of thumb is that unless the value contains only lower or upper case letters in the range a-z you must put quotation marks around the value. In fact, unless you have extreme file size requirements it is a very very good idea to always put quote marks around your attribute values. It is never wrong to do so, and very often it is absolutely necessary.
Line 71, Column 5: character "<" is not allowed in the value of attribute "id"   </div>✉It is possible that you violated the naming convention for this attribute. For example, id and name attributes must begin with a letter, not a digit.
I have absolutely no clue what it is asking of me. Doesn't it have to be that way? Also why when I go to "Line 71" that isn't where this actual code is? Is there some special way to decipher this?