[css-page-3] page context counter scopes are described incorrectly

[faceless2]

(migrated from first half of #3520 to make a single point per issue, and updated)

css-page-3 has the following language:

If a counter is reset or incremented within the page context, it is in scope for all page-margin boxes and obscures all counters of the same name within the document.

This implies the counter falls out of scope at the end of the page context - which means a counter incremented in the page context cannot be referenced from within the document body, or indeed within another page context. This clearly isn't the intention, otherwise counter-increment: page in the page context couldn't work.

A qualifier was suggested when this was first raised by @fantasai, but it seems to have eroded over time.

I think the intended meaning is if a counter is incremented for the first time - because incrementing any counter without a previous reset will create a new counter.

Should this instead read "if a counter is created within the page context"?

"Created" is the specific term used in css-lists for when a counter is reset or set/incremented without a previous reset. With an implied user-agent rule of

:root { counter-reset: page 0 }

then the "page" counter is not being created in the margins, so can be shared across the whole document.

[faceless2]

Two things I want to add to this:

First, I need to make clear that the above interpretation "if a counter is created within the page context" is absolutely critical to making page-context counters work. Without it, it's impossible to use any counter other than "page" in the page margin; it would always fall out of scope at the end of the page context (in the example below, the "preface" counter would always have the value "1"). This clearly isn't the intention. The "page" counter only works because it's behaviour is explicitly described.

Second, in order for any counter other than "page" to be used in the page context, it is fairly important that counters can be created before the first page. An example:

:root {
    counter-reset: preface 5;
}
@page {
   @top-center {
      counter-increment: preface 1;
      content: counter(preface);
   }
}

If the :root element is created and the "preface" counter initialized before the first page is created, the counter at the top of the first page will be "6", the next one "7" and so on.

But if the :root element is created after the first page, the counter at the top of the first page will be "1", the second page "6" and the third page "7" and so on. Because the counter doesn't exist when the first page is created, the "preface" counter is created in the first page context, then falls out of scope. On subsequent pages, the counter created by the :root element is in scope and it increments as normal.

I think it makes sense to add a line to the first block of section 6.1 that:

Counters created on the root element are in scope for all page-margin boxes