Google Sheets – Can Conditional Formatting Reference Another Sheet?

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I needed to check which indexes exist on another sheet.
So for the A:A range, I made conditional formatting with the following formula:

=COUNTIF('Other Sheet'!A:A;A1)

That had a red error outline.
Then I filled column X with ='Other Sheet'!A:A and used the formula

=COUNTIF(X:X;A1)

It worked instantly.
What am I doing wrong?

It can, but you need to wrap it in INDIRECT:

=COUNTIF(INDIRECT("'Other Sheet'!A:A"); A1)