CSS can keep count of numbers without writing any additional JavaScript.

It does this by taking note the amount of time a CSS block affects a page then incrementing the counter for that block if the counter-increment rule is implemented.

For example:

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input:invalid {
  counter-increment: invalid-count;
}

With no JavaScript at all, CSS understands that whenever there is an invalid element, it should increment the count for the invalid-count identifier.

Structure of CSS counters

CSS counter is a CSS rule that specify the increment-counter property and an indentifier for the counter to increment.

To know how many HTML exists in a page:

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h1 {
  increment-counter: total-h1-counter;
}

Since CSS would match every h1 on the page, the increment-counter would be executed, thereby increasing the count for the total-h1-counter.

Getting back to 0

A counter can go back to the 0 state by declaring the counter-reset: counter-identifier CSS rule

A use case here would be always reseting the number of total-h1-counter on the page when the body of the document is matched, but incrementing when H1s are matched.

The CSS below helps to achieve that:

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body {
  counter-reset: total-h1-counter;
}

h1 {
  counter-increment: total-h1-counter;
}

Rendering counter on a page.

We can use CSS Generated Content to make the value of a counter visible on the page. We can achieve this using pseudo elements(::before, ::after) through the content property.

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<h1>The first</h1>
<h1>The Second</h1>
<h1>The third</h1>
<h1>The fourhtb</h1>

<section>There are</section/>

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body {
  counter-reset: total-h1-counter;
}
h1 {
  counter-increment: total-h1-counter;
}
section:after {
  content: " " counter(total-h1-counter) " H1 tags on the page";
  font-weight: bold;
  color: tomato;
}

Salut!