[css-page-3] Specify how to handle negative page margins

[dholbert]

(Related to #8335 , but now for margin...)

I don't think the css-page-3 spec ( https://w3c.github.io/csswg-drafts/css-page-3/ ) defines what to do (if anything) with negative @page { margin: ...} values (though I may have overlooked it).

In practice:

• Chromium seems to honor negative @page margins, effectively inflating the page content area beyond the bounds of the sheet of paper on the sides that have negative margins. (So, content gets clipped on those sides, generally.)
• Firefox clamps negative @page margin to 0 (or rather, to the printer's writable page area) with the intent of keeping page content on the sheet. (as of https://bugzilla.mozilla.org/show_bug.cgi?id=1665001 )

[dholbert]

See also https://bugzilla.mozilla.org/show_bug.cgi?id=1679041 which was filed 2 years ago by someone who happened to depend on the Chrome behavior (and the behavior that Firefox used to have before https://bugzilla.mozilla.org/show_bug.cgi?id=1665001 )

[MurakamiShinyu]

There are certain use cases of negative page margins.

• negative page margins are used to prevent unwanted page breaking. This is necessary unless we can use break-inside: never (discussed in [css-break-4] Should fragmentation of block-level replaced-elements be configurable? ("object-slice") #3404).

• negative page margins can be used to position images fitting to the bleed edges.
  Example:

  @page  {
    size: A4; /* 210mm 297mm */
    marks: crop;
    bleed: 3mm;
    margin: -3mm;
  }
  
  img {
    display: block;
    width: 100vw; /* = 216mm */
    height: 100vh; /* = 303mm */
  }