Reconcile BCD and caniuse for `page-break` properties results

[ddbeck]

We set an imperfect compute_from value in #1802 to align with caniuse's https://caniuse.com/css-page-break information. This is OK for now because we get the gist correctly: it's only working in Chromium.

But the version numbers diverge wildly (Chrome 1 versus Chrome 108). Something's not right; tests are probably needed to figure out if BCD or caniuse has it right, then PR(s) against both data sources to resolve the discrepancy.

Read these before doing anything more:

• Add CSS page breaking feature #331
• What is fully supported for CSS page-break properties? Fyrd/caniuse#7045
• page-break-{before,after}: avoid not supported in Safari and Firefox mdn/browser-compat-data#22987
• Adds additional compat keys to page-breaks and column-breaks #1802

[tidoust]

Stumbling upon this after having tried to convert a web page into a book for a week-end project of mine.

In Fyrd/caniuse#7045, the rationale includes:

The spec mentions "A conforming user agent may interpret the values 'left' and 'right' as 'always'. "

That's true for the page-break-* properties but I don't think it is for the break-* properties: CSS 2 indeed says that for the now deprecated page-break-* properties. But the CSS Fragmentation module that defines the break-* properties and values that replace the page-break-* properties no longer has that clause. The links in BCD and web-features are against the CSS Fragmentation module, not against CSS 2.

In practice, both Chrome and Firefox (I didn't test on Safari) interpret break-before: left and break-before: right as break-before: page (same for break-after), which the CSS Fragmentation module does not suggest is conforming behavior unless I missed something in the spec.

All in all, it seems to me both sources are wrong:

• Support data in BCD comes from the BCD collector, which considers that the left and right values are supported because the browsers don't reject the values.
• Support data in Can I Use seems grounded on past page-break-* properties but behavior of these deprecated properties is now defined in terms of the break-* properties.

Web-features asserts that the Page breaks feature is widely available as a result. That's only correct if we consider that avoid page are the only values that matter.