cdn.jsdelivr.net挂了备用方案
fastly.jsdelivr.net
testingcf.jsdelivr.net
gcore.jsdelivr.net
quantil.jsdelivr.net
切换上面任意域名地址
切换前
https://cdn.jsdelivr.net/npm/bootstrap@4.4.1/dist/css/bootstrap.min.css
切换后
https://fastly.jsdelivr.net/npm/bootstrap@4.4.1/dist/css/bootstrap.min.css