editor.on('toHtml', escapeStyleContent, null, null, 1);
editor.on('toDataFormat', escapeStyleContent, null, null, 1);

function escapeStyleContent(event) {
  let html = event.data.dataValue;
  let parts = html.split('</style>');
  for (let i = 0; i < parts.length - 1; i++) {
    let part = parts[i];
    let styleStart = part.lastIndexOf('<style');
    if (styleStart >= 0) {
      let styleContentStart = part.indexOf('>', styleStart + 6) + 1;
      if (styleContentStart > 0) {
        let styleContent = part.substring(styleContentStart);
        styleContent = styleContent.replaceAll('<', '\\3C');
        parts[i] = part.substring(0, styleContentStart) + styleContent;
      }
    }
  }
  event.data.dataValue = parts.join('</style>');
}

const domParser = new DOMParser();
function escapeStyleContent(event) {
  let html = event.data.dataValue;
  const isFragment = !html.trimEnd().endsWith('</html>');
  try {
    const doc = domParser.parseFromString(html, 'text/html');
    let modified = false;
    doc.querySelectorAll('style').forEach(style => {
      const styleContent = style.textContent;
      style.textContent = styleContent.replaceAll('<', '\\3C');
      modified = modified || (styleContent !== style.textContent);
    });
    if (modified) {
      event.data.dataValue = isFragment ? doc.body.innerHTML : doc.documentElement.outerHTML;
    }
  } catch (e) {
    console.warn('Failed to escape the style tags from the given HTML string: ' + html, e);
  }
}

require(['deferred!ckeditor'], function(ckeditorPromise) {
  ckeditorPromise.done(function(ckeditor) {
    ckeditor.on('instanceCreated', function(event) {
      event.editor.on('toHtml', escapeStyleContent, null, null, 1);
      event.editor.on('toDataFormat', escapeStyleContent, null, null, 1);
    });
  });

  const domParser = new DOMParser();
  function escapeStyleContent(event) {
    let html = event.data.dataValue;
    const isFragment = !html.trimEnd().endsWith('</html>');
    try {
      const doc = domParser.parseFromString(html, 'text/html');
      let modified = false;
      doc.querySelectorAll('style').forEach(style => {
        const styleContent = style.textContent;
        style.textContent = styleContent.replaceAll('<', '\\3C');
        modified = modified || (styleContent !== style.textContent);
      });
      if (modified) {
        event.data.dataValue = isFragment ? doc.body.innerHTML : doc.documentElement.outerHTML;
      }
    } catch (e) {
      console.warn('Failed to escape the style tags from the given HTML string: ' + html, e);
    }
  }
});