Q: I can't open a certain webpage in my macbook air

Do you have any anti-virus or security software installed on your Mac?

Website opens on this Mac without any problem.

Safari 9 (El Capitan): If Safari can’t open a website

https://support.apple.com/kb/PH21431?viewlocale=en_US&locale=en_US

If Safari doesn‘t load a page or webpage items are missing

https://support.apple.com/en-us/HT204098

If Safari is slow, stops responding, quits unexpectedly, or has other issues

https://support.apple.com/en-us/HT203353