The answer is Ajax: Asynchronious javascript and XML.
The purpose of ajax is exactly what you said. Google ajax for more info.
题
When you enter your credentials on reddit login page, it first checks their validity and if they're valid only then it redirects, otherwise the fields are updated with a "wrong password" message.
I'm trying to understand how the submit button validates the credential without actually submitting the form, and thus, reloading the page.
解决方案
The answer is Ajax: Asynchronious javascript and XML.
The purpose of ajax is exactly what you said. Google ajax for more info.