[CSSGapDecorations] Add pseudo-element alternative

CSSGapDecorations/explainer.md

@@ -56,6 +56,7 @@ content location of future work and discussions.
     - [Logical properties for flex and masonry containers](#logical-properties-for-flex-and-masonry-containers)
   - [Considered alternatives](#considered-alternatives)
     - [Alternative 1: 2021 draft specification](#alternative-1-2021-draft-specification)
+    - [Alternative 2: Using pseudo-elements](#alternative-2-using-pseudo-elements)
   - [References \& acknowledgements](#references--acknowledgements)
 
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@@ -456,6 +457,41 @@ grid lines, and (b) simplifying the model for fine-tuning segment placement. We
 also believe the proposal in this explainer offers developers more flexibility
 even absent support for gap decoration images; see Scenario 3 for one example.
 
+### Alternative 2: Using pseudo-elements
+
+An alternative approach to `column-rule-*` and `row-rule-*` properties would be
+to introduce pseudo-elements representing gaps, for example:
+
+```css
+.container {
+  display: grid;
+  grid-template: auto / auto;
+  gap: 5px;
+}
+.container::column-gaps {
+  background: red;
+  width: 1px;
+}
+.container::row-gaps {
+  background: blue;
+  width: 1px;
+}
+```
+
+In some ways, this would be more powerful, as it would allow for more
+flexibility for what can be placed in the gaps.
+
+However, this approach also comes with drawbacks. Varying gap decorations over a
+container becomes much harder. One might imagine a `::row-gaps::nth(even)`
+pseudo selector to style every other row gap. However, certain container types
+such as grid can automatically generate rows and columns depending on their
+contents. That means we don't know until layout time how many such pseudo styles
+we need to produce, which creates a wrong-way dependency between layout and
+style. It would also mean that, for large containers, we would incur the costs
+of calculating and storing styles for every single gap. That would be a large
+overhead to absorb, especially considering that the more common case is to have
+at most a single decoration style for a given container.
+
 ## References & acknowledgements
 
 Many thanks for valuable feedback and advice from: