If you have a list inside a container with an id, you only need that id to isolate the list (as long as other kinds of pages don't also have the same id for different purposes). Exactly what selectors are you using for your rule? Something like
#productDescription ul {}
or
#productDescription li {}
should work, depending on exactly what you are trying to do.
If you want to distinguish between lists inside one product description, or between different products' lists, you could add a class to that list, like
HTML Code:
text text text text
<ul class="doohickyList">
<li>text text </li>
<li>text </li>
<li>text text text </li>
</ul>
text text
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