`::selection` is eager in Servo but not in Gecko?

[Loirooriol]

Gecko has 4 eager pseudo-elements (https://searchfox.org/firefox-main/rev/ac15fb210057449549c7a15ed93286083a28d93d/servo/components/style/gecko/pseudo_elements.toml#28,32,40,43)

• ::after
• ::before
• ::first-letter
• ::first-line

Servo has 3, with #320 adding ::first-letter as a 4th.

stylo/style/servo/selector_parser.rs

Lines 45 to 47 in ecf17b5

	After = 0,
	Before,
	Selection,

We don't support ::first-line yet. But why is ::selection eager for Servo and not for Gecko?