使用requests+BS爬取B站视频名称时，提取不到想要的元素？

import requests
from bs4 import BeautifulSoup

url='https://search.bilibili.com/all?keyword=%E4%B8%AD%E5%9B%BD%E4%BA%A4%E9%80%9A%E4%BA%8B%E6%95%85%E5%90%88%E9%9B%86&page=1&order=pubdate'
headers={
    'Cookie':'fts=1508585882; sid=8qyd4ip9; buvid3=0980BD3F-2692-44C3-BD9F-2835C27D632326558infoc; UM_distinctid=15f3eb9d59010d-0f8304e5b36d25-791238-144000-15f3eb9d591219; pgv_pvi=1610170368; rpdid=olxqpxmpmmdoswqsqsqqw; finger=edc6ecda; fold=false',
    'User-Agent':'Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/62.0.3202.62 Safari/537.36'
}
r=requests.get(url,headers=headers)
soup=BeautifulSoup(r.text,'lxml')
lis=soup.find_all('li',_class='video matrix ')
print(lis)

但打印soup.pretiffy()时是有class='video matrix '元素的？