There is a whole continuum of remote server "up" detection - from mere connectivity, through to accurate and timely responses. A machine may be in several states and conditions, and depending on quite what you are looking for, some of which can be:

• single-user
• multi-user
• port 80 accepting
• http://domain responding
• http://domain/file responding
• http://domain/file responding quickly enough

It may also be that the site content is unpredictable, so a specific URL may not be something you want to check.

Then there is a question of whether one of the unacceptable conditions might change if you retry. For example, instantaneous loads may indicate a very poor response time, but with a perfectly acceptable average.