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when you open https website the web site sent SSL certificate and the web browser there some identification on SSL Certificate(like who issued the certificate,Expiration Date,URL match to the issed Certificate) after the browser perform basic validation and then contact the CA authority server (through CRL points) to make sure the certificate is still good.
if the browser do not determine one of the following is not okay you will see the below warning image
1) Who issued the certificate
2) the SSL Cerificate has pass expiration date
3) the issued certifcate Url and the current Url is differnt