Frames with focusable content must not have tabindex=-1
Moderateframe-focusable-content
What it checks
Ensure <frame> and <iframe> elements with focusable content do not have tabindex=-1
How to fix it
Review and fix this accessibility issue following WCAG guidelines
WCAG
Success criteria 2.1.1. See the WCAG 2.2 quick reference.
More detail: Deque University's page on this rule.