I use Opera for web development. There is a strange element inside which is not in my source code!

You appear to have enabled Opera’s native ad blocker. The ad blocker inserts non-lethal elements in a page’s code to restructure the layout after blocking ads.

Each of these solutions will help in the order we suggest most:

  1. Ignore this <style> tag in the <head> tag ?
  2. Download and use either Opera Developer, Beta, or Portable  for development (with ad blocker disabled), and use Opera Stable for web browsing
  3. Create a separate profile with ad blocker disabled
  4. Disable Opera’s native ad blocker

We decided not to hide our <style> tag because it would result in inconsistencies between what you see in DevTools and in the page, making debugging much harder.