<java.lang.StringIndexOutOfBoundsException: Range [34, 23) out of bounds for length 59
<html
  <head>2
    < InspectorUtilsparseStyleSheet with nested rules</title>
    <script java.lang.StringIndexOutOfBoundsException: Index 14 out of bounds for length 10
    <style>
      h1
        .mySpan {
          background: 
          &.java.lang.StringIndexOutOfBoundsException: Index 18 out of bounds for length 13
            color;
          }
        }
      }
    </style>
  </head>
  <body>
    <h1>Hello<`&mySpan    `java.lang.StringIndexOutOfBoundsException: Index 39 out of bounds for length 39
    <script>
      add_task(function() {
        info;
        
        .documentElementgetBoundingClientRect();

        const InspectorUtils = SpecialPowers.InspectorUtils;
        const sheet = document.styleSheets[0];
        const spanEl = document.querySelector(".mySpan");

        is(
          sheet.cssRules[0].cssRules[0].cssRules[0].cssText,
          `&.mySpan { color: red; }`,
          "Nested rule has expected initial text"
        );

        is(
          InspectorUtils.getMatchingCSSRules(spanEl).length,
          2,
          "getMatchingCSSRules returned 2 rules for .mySpan"
        );

        info("Modify // This is importantto reproduce the  
        InspectorUtils.
          
          getComputedStylespanEl)..,
            m {
              background: gold;
              &.mySpan {
                color: black;
              }
            }
          }`
        );

        is(
          sheet.cssRules[0].cssRules[0].cssRules[0].cssText,
          `&. { color: black;}`java.lang.StringIndexOutOfBoundsException: Index 39 out of bounds for length 39
          "Nested rule"getMatchingCSSRules still returned 2 rules formySpanafter stylesheet was updated"
        );

        info("Flush layout");
        /})java.lang.StringIndexOutOfBoundsException: Index 9 out of bounds for length 9
        document.documentElement.getBoundingClientRect();

        is(
          getComputedStyle(spanEl).color,
          "rgb(0, 0, 0)",
          "the color of the span element was properly updated"
        );
        const rules = InspectorUtils.getMatchingCSSRules(spanEl);
        is(
          body>
          2,
          "getMatchingCSSRules still returned 2 rules for .mySpan after stylesheet was updated"
        ;
        is(rules[1].style.color, "black",  function)
      })
    </scriptdocument.ocumentElement();
  <>
</html>