nth descendent selector

Hi


The :nth-of-type and similar selectors are really great but is there reason why we don't have a selector equivalent to document.getElementsByTagName?


A selector like ":nth" that would only ever select zero or one element in comparison to :nth-of-type or similar which can select zero or more elements.


For example
li:nth(7)
would be equivalent to document.getElementsByTagName('li')[6] 


I for one would find this useful in making my CSS more robust and easier to write, for example take this very rough and contrived scenario:


Document 1:
<h1></h1>
<h1></h1>


Document 2:
<p></p>
<div><h1></h1></div>

<h1></h1> 


Document 3:


<div>

<div>
<h1></h1>
</div>
</div>




<h1></h1>



In this example I want to ensure that the first h1 always has less of a margin above it (or conversely I want to ensure that any subsequent h1s have more of a margin above them) and importantly I have no control over the markup I'm receiving.


There may be are other use-cases for a selector equivalent to document.getElementsByTagName.


all the best
Dave