input: Write a python function to make a webpage mobile-friendly.
  assert 'width=device-width, initial-scale=1' in make_webpage_mobile_friendly('http://example.com')
  assert 'max-width:100% !important' in make_webpage_mobile_friendly('http://example.com')
  assert '<meta content="width=device-width, initial-scale=1" name="viewport"/>' in make_webpage_mobile_friendly('http://example.com')
output: def make_webpage_mobile_friendly(url):
      import requests
      from bs4 import BeautifulSoup
  
      response = requests.get(url)
      soup = BeautifulSoup(response.text, 'html.parser')
  
      # Add viewport meta tag if not present
      if not soup.find('meta', attrs={'name': 'viewport'}):
          viewport_tag = soup.new_tag('meta', attrs={'name': 'viewport', 'content': 'width=device-width, initial-scale=1'})
          soup.head.append(viewport_tag)
  
      # Add CSS to make images and videos responsive
      style_tag = soup.new_tag('style')
      style_tag.string = """
      img, video {
          max-width: 100% !important;
          height: auto !important;
      }
      """
      soup.head.append(style_tag)
  
      return str(soup)