How create CSS Selector which selects the Xn tag including descendants - css

Question

Say I have the following DOM tree:
<div class="box">
<ul class="gallery">
<li id="1">text</li>
<li id="2">text</li>
<li id="3">text</li>
<li id="4">text</li>
<li id="5">text</li>
<li id="6">text</li>
<li id="7">text</li>
<li id="8">text</li>
<li id="9">text</li>
</ul>
<div id="random"></div>
<ul class="gallery">
<li id="10">text</li>
<li id="11">text</li>
<li id="12">text</li>
<li id="13">text</li>
<li id="14">text</li>
<li id="15">text</li>
<li id="16">text</li>
<li id="17">text</li>
<li id="18">text</li>
<li id="19">text</li>
<li id="20">text</li>
</ul>
</div>
I want to create a CSS selector that will pick every 6th <li> tag under the div with the class "box". But I need the selector to take into account the entire <li> tags in the page and not to count them per <ul> tag. So in the end, the selector should pick the <li> with IDs 6,12,18. Currently I was only able to create a selector that picks IDs 6 & 15 when I used:
div.box li:nth-of-type(6n)
Notice 1: the IDs numbers are only added for reference. In reality the <li> tags don't have a class or an ID.
Notice 2: the number of <li> tags in each <ul> tag varies from site section to site section. Sometimes there can be even a 3rd and a 4th </ul> with more <li> tags.
Notice 3: I don't have control over the hard-coded HTML, which means I cannot unify tags, add IDs or CSS classes, etc. The selector will be called from an external JS file. While I can edit the DOM with jQuery after the page loads, I prefer to avoid such a solution to make the selector easier to handle.
Thanks

Answer 1

Generally Agree Impossible, except...
I basically agree with Sych and Fabrício that it is not currently possible to do as a pure CSS solution. However, there are always some exceptions, depending on actual html structure.
In your case, there may be an exception, but it does depend on the full constraints of your situation.
When Would it Be Possible?
Based off your given code, if these two constraints are true, then you can still get what you want via CSS:
All ul items that are direct children of .box are of class .gallery.
All gallery groups (except perhaps the very last one) consist of exactly nine li elements (both groups in your example do, but I don't know if that was coincidence or how you are actually setting up your code).
If the above two factors in your html are true, then this code gets what you want (using color to show selection here):
ul.gallery:nth-of-type(2n+1) li:nth-of-type(6n) {
color: red;
}
ul.gallery:nth-of-type(2n+2) li:nth-of-type(6n+3) {
color: red;
}
You can see it works on the code you gave in this fiddle, then you can see it continues to work given an expansion of the html as this fiddle shows, even if the final list is short of nine as both this fiddle and this fiddle shows, but it will fail if the .gallery varies in length at some midpoint of the sequence, as seen in this fiddle (notice how the last two selected texts are not 6 apart from each other because the second to last .gallery has only 7 items).
The Overarching Principle
So in general, if your dynamic html is output in some type of a regular pattern as demonstrated here, then it can open up the possibility of a pure css solution along the lines of that given. It is when the dynamic generation is also fully random (so in your case if either #1 or #2 condition above is not guaranteed true) that a pure css solution is impossible.

Answer 2

CSS does not provide such scope, it only provides traversing "deeper in to the DOM" tree. It does not even have a parent element selector.
If you are in jQuery environment, you can write your own selector, call it, say, ":nth-from-top(n)" that will work using jQuery's DOM traversing functions.
Note, that this type of selector will be much slower, because it cannot take advantage of the performance boost provided by the native DOM methods.

Answer 3

nth-child and nth-of-type match based in the element's position relative to its siblings only.
As far as I know there's currently no CSS-only solution for that unless all lis had the same parent. You will have to add a class to every 6th element or use some JavaScript.
So, constraining the answer to CSS selectors only without altering the markup and without hardcoding the nth start indexes: impossible. I'd like to be proven wrong though.
Looking by the bright side, adding a class will provide better selector performance. nth-child is already considered inefficient, now if what you want would be possible it'd mean that browsers would be forced to recursively evaluate selectors and count matches each time the DOM is updated. Though this would be terrible performance wise, I believe it'd still be possible to implement through new "scoped" nth selectors a la CSS Counters. Just food for thought.