parentStyleSheet returns the stylesheet object in which the current rule is defined.
var stylesheet = cssRule.parentStyleSheet
stylesheet
is a stylesheet object.if ( bgRule.parentStyleSheet != mySheet ) { // alien style rule! }
See Gecko DOM Reference:event#DOM_styleSheet_Object for more information about the stylesheet object interface.
DOM Level 2 Style - cssRule
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https://developer.mozilla.org/en-US/docs/Web/API/CSSRule/parentStyleSheet