CSSFontFaceRule

@font-face {
  font-family: "MyHelvetica";
  src:
    local("Helvetica Neue Bold"), local("HelveticaNeue-Bold"),
    url("MgOpenModernaBold.woff2");
  font-weight: bold;
}

#log {
  height: 200px;
  overflow: scroll;
  padding: 0.5rem;
  border: 1px solid black;
}

<pre id="log"></pre>

const logElement = document.querySelector("#log");
function log(text) {
  logElement.innerText = `${logElement.innerText}${text}\n`;
  logElement.scrollTop = logElement.scrollHeight;
}

const myRules = document.getElementById("css-output").sheet.cssRules;
for (const rule of myRules) {
  if (rule instanceof CSSFontFaceRule) {
    log(`this: ${rule}`);
    log(` cssText: ${rule.cssText}`);
    log(` parentRule: ${rule.parentRule}`);
    log(` parentStyleSheet: ${rule.parentStyleSheet}`);
    log(` type: ${rule.type}`);
    log(` style: ${rule.style}`);
  }
}