input: Write a python function to find the most frequent word in an HTML file. 
  assert most_frequent_word('example1.html') == 'the'
  assert most_frequent_word('example2.html') == 'and'
  assert most_frequent_word('example3.html') == 'to'
output: def most_frequent_word(filename):
      from bs4 import BeautifulSoup
      from collections import Counter
      import re
  
      with open(filename, 'r', encoding='utf-8') as file:
          soup = BeautifulSoup(file, 'html.parser')
  
      text = soup.get_text()
      words = re.findall(r'\b\w+\b', text.lower())
      word_counts = Counter(words)
      most_common_word, _ = word_counts.most_common(1)[0]
  
      return most_common_word