Function style_or_page_rule_to_css 

fn style_or_page_rule_to_css(
    rules: Option<&Arc<Locked<CssRules>>>,
    block: &Locked<PropertyDeclarationBlock>,
    guard: &SharedRwLockReadGuard<'_>,
    dest: &mut CssStringWriter,
) -> Result

Serialize a page or style rule, starting with the opening brace.

https://drafts.csswg.org/cssom/#serialize-a-css-rule CSSStyleRule

This is not properly specified for page-rules, but we will apply the same process.