IANAL, but I think this would be borderline. It's arguably "fair use", and sounds like it meets the exception specifically provided in copyright law for "review and criticism". But copying an entire web page is pushing it. I'd ask for permission, and if someone says no, don't use their site.
What you CAN do is, if you see something on a web site that you think is a good idea, make your own sample web page that embodies that good idea, but that is not copied from the original. For example, if you think the collection of colors used on a particular site is particularly attractive, easy to read, eye-catching, whatever, you could mock up a web page using those same colors but your own text. Etc.