HTML is Single Inheritance!

I was trying my ideas tree. It seems to work nicely, but I found one question that is somewhat a revelation:

What if I want to link one node to another node? The layout I did is that one parent has many childs, but what if I want a child to come from many parents? I have never thought about that!

The only possible answer is to make that node to hold a reference to that parent. This reminds me of the problem of references that are assigned at runtime: they can't be confirmed at compile time. Why is it bad? because your code can work wonders when you tested it, but when your clients do their testings....

OOP depends on Type Checking to help to "detect" those possible problems, but what about HTML? Now, EcmaScript is getting more and more necessary.