Test nested at-rules. On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE". rules = document.styleSheets[1].cssRules PASS rules.length is 2 PASS rules[0].type is CSSRule.STYLE_RULE PASS rules[1].type is CSSRule.MEDIA_RULE PASS rules[1].cssRules.length is 2 PASS rules[1].cssRules[0].type is CSSRule.MEDIA_RULE PASS rules[1].cssRules[0].cssRules.length is 3 PASS rules[1].cssRules[0].cssRules[0].type is CSSRule.FONT_FACE_RULE PASS rules[1].cssRules[0].cssRules[1].type is CSSRule.PAGE_RULE PASS rules[1].cssRules[0].cssRules[2].type is CSSRule.STYLE_RULE PASS rules[1].cssRules[0].cssRules[0].parentStyleSheet is document.styleSheets[1] PASS rules[1].cssRules[0].cssRules[1].parentStyleSheet is document.styleSheets[1] PASS rules[1].cssRules[0].cssRules[2].parentStyleSheet is document.styleSheets[1] PASS rules[1].cssRules[1].type is CSSRule.MEDIA_RULE PASS rules[1].cssRules[1].cssRules.length is 1 PASS rules[1].cssRules[1].cssRules[0].type is CSSRule.MEDIA_RULE rules[1].cssRules[1].insertRule('@media all { @page :left { top: 0; } }', 1) PASS rules[1].cssRules[1].cssRules.length is 2 PASS rules[1].cssRules[1].cssRules[1].type is CSSRule.MEDIA_RULE PASS rules[1].cssRules[1].cssRules[1].cssRules[0].type is CSSRule.PAGE_RULE PASS getComputedStyle(document.getElementById('t0')).content is "\"APPLIED\"" PASS getComputedStyle(document.getElementById('t1')).content is "\"UNTOUCHED\"" PASS successfullyParsed is true TEST COMPLETE