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Simple VisualForce pages render reasonably well in Salesforce1 as they are, but if this is general viewing via browser you'll probably find things aren't looking great. If I was going to do this I'd use Bootstrap - it's designed for creating responsive pages easily and wouldn't have an overhead as large as other frameworks.
*Note: This will be present if you navigate to /one/one.app, whether that be the Salesforce1 mobile app, in your mobile browser, or a desktop/laptop browser navigated to /one/one.app
You can also use Apex to identify the user agent