Function installStylesForDoc

src/style-installer.js:36–71

(
  ampdoc,
  cssText,
  cb,
  opt_isRuntimeCss,
  opt_ext
)

 * @return {!Element}
 */
export function installStylesForDoc(
  ampdoc,
  cssText,
  cb,
  opt_isRuntimeCss,
  opt_ext
) {
  const cssRoot = ampdoc.getHeadNode();
  const style = insertStyleElement(
    cssRoot,
    maybeTransform(cssRoot, cssText),
    opt_isRuntimeCss || false,
    opt_ext || null
  );

  if (cb) {
    const rootNode = ampdoc.getRootNode();
    // Styles aren't always available synchronously. E.g. if there is a
    // pending style download, it will have to finish before the new
    // style is visible.
    // For this reason we poll until the style becomes available.
    // Sync case.
    if (styleLoaded(rootNode, style)) {
      cb(style);
      return style;
    }
    // Poll until styles are available.
    const interval = setInterval(() => {
      if (styleLoaded(rootNode, style)) {
        clearInterval(interval);
        cb(style);
      }
    }, 4);
  }
  return style;
}

/**
 * Creates the properly configured style element.