Counting DOM elements with CSS - not working consistently - css

Question

As part of some performance testing, I want to see how many DOM elements there are within a page at any given time.
Initially I used the console and manually typed: document.getElementsByTagName('*').length
But being that elements are dynamically added and removed, I wanted a way to show this automatically without having to do this manually.
Using CSS counters seemed to be the easiest way for me to do this:
(Here's a demo: see the yellow box in the top right)
html {
counter-reset: elems;
counter-increment: elems;
}
html * {
counter-increment: elems;
}
body:after {
content: counter(elems) ' elements';
position: absolute;
top:0;
right: 0;
padding: 0.5rem;
background-color: yellow;
z-index: 1000000000; /* make sure display appears on top */
}
html + head + body = 3... +
A section with 4 divs = 5..... 3 + 5 = 8
<section>
<div>div1</div>
<div>div2
<div>div3</div>
</div>
<div>div4</div>
<section>
The problem is that this technique is not working for some more complex websites, and i'm guessing that it's because it's somehow naive.
Take for instance wikipedia's main page - I used the browser inspector to add the above css-counter css and that produced "888 elements"
However when I used the console document.getElementsByTagName('*').length - it returned 975 elements.
So my question is:
What is causing this huge discrepancy in the number of DOM elements, is there something I haven't taken into account?
Can this be done reliably with CSS?

Answer 1

The reason you get different counts is because your counter-increment: elems; rule is lower priority than some existing counter-increment rules in Wikipedia's stylesheet, so there are elements which increment other counters (one example is named "listitem") but not yours.
You'd have to make your selector more specific and override Wikipedia's, or add the website's own counter (or write some JavaScript to add your own counter in the existing counter-increment rules, to avoid breaking the existing style).

Answer 2

What is causing this huge discrepancy in the number of DOM elements,
is there something I haven't taken into account?
Yes, elements which have display:none applied are ignored by css counters.
From the spec:
An element that does not generate a box (for example, an element with
display set to none, or a pseudo-element with content set to none)
cannot set, reset, or increment a counter. The counter properties are
still valid on such an element, but they must have no effect.
Demo:
html {
counter-reset: elems;
counter-increment: elems; /* Include the html element as well */
}
html * {
counter-increment: elems;
}
body:after {
content: counter(elems) ' elements';
position: absolute;
top:0;
right: 0;
padding: 0.3rem;
background-color: lightyellow;
z-index: 1000000000; /* make sure display appears on top */
}
.hidden {
display: none;
}
html + head + body = 3... +
A section with 4 divs = 5..... 3 + 5 = 8
<section>
<div>div1</div>
<div>div2
<div>div3</div>
</div>
<div>div4</div>
<div class="hidden"></div>
<div class="hidden"></div>
<div class="hidden"></div>
<div class="hidden"></div>
<div class="hidden"></div>
<div class="hidden"></div>
<section>
On the other hand document.getElementsByTagName('*').length WILL count elements with display:none
This may only be one of the differences which I hadn't taken into account, but I'd say it's probably the main culprit as to the discrepancy.