Clarifying what sorts of outcomes are realistic here:

(In reply to Hildo Bijl from comment #0)

Expected results:

The borders should all ... remain visible

Let's focus on this^ request; I think this is the least-controversial expectation here.

To be
precise, when a border of "0.8px" is scaled by a factor "0.5", the resulting
border should look exactly like it would've looked when given the width
"0.4px" without scaling.

This is a reasonable intuition but it's a non-goal and is not actually how borders work in any browser.

In the absence of transforms, I think all browsers snap borders to whole-number display-pixel-values (at least, they do for borders less than 1px) -- so e.g. 0.4px will just round up to one display-pixel (and so will 0.01px), at computed-value-time (i.e. and it actually reserves & occupies that much layout space, rather than the specified tiny-fraction-of-a-pixel). This is interoperable and required for compatibility; you can try e.g. data:text/html,<div style="border: 0.01px solid black">Hi in various browsers and see that it occupies a full display-pixel (e.g. it's equivalent to border:1px solid black on a traditional non-HiDPI display resolution).

In contrast, transform:scale(0.5) is a paint-time effect and we don't get the opportunity to make the border specially-reserve-and-occupy a full display pixel of layout space when we apply a scaling transform. Chrome seems to use antialiasing to paint the scaled-down partial-pixel-width-border, whereas Firefox seems to still try to paint a whole number of pixels (possibly 0); we round down to 0px for sufficiently-small scales / sufficiently-thin borders. This outcome is not-great; ideally we should allow ourselves to paint fractional-width-borders as antialiased partial-pixels when we're scaling them, I think.

Also FWIW, I can see that the original testcase here renders without a top black border in old Firefox nightlies e.g. Firefox Nightly 48.0a1 (2016-03-19), but the whole black border paints in current Firefox Nightly (2022-06-21).

But if I reduce the scale to e.g. 0.2 or 0.5, I can still reproduce (the black border disappears).

I've got a clearer/more-comprehensive testcase on my dupe bug, which I'll repost here for clarity/convenience.

screenshot of reduced testcase 1 in Firefox vs Chrome: https://bugzilla.mozilla.org/attachment.cgi?id=9282271

The severity field for this bug is relatively low, S3. However, the bug has 5 duplicates.
:emilio, could you consider increasing the bug severity?

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Other browsers are going to match our behavior here and it's not clear there's a terribly-better behavior around. We could make WebRender force 1 dev px borders at paint-time too, but...