Host unavailable - This just means that the machine that hosts the site is down. It may just be unhooked from the network, or it may be down for maintainence. If this doesn't work, try again later as usual. You can either wait a while and try later, as it says. Your browser supports all sorts of neat stuff like forms.
All good browsers support the industry standard form elements.
The error occurs when someone writes a web page with a form and there is either an error in the form or they decided to get creative and add non-standard elements or features. Network connection was refused by the server - Usually this just means that the server is too busy to handle one more user, and that it is not set up to tell you so. As a result, you just see this generic message. It means that something is wrong with the address. Maybe the server doesn't recognize the document you asked for, maybe the document doesn't exist, or maybe you don't have authorization to view the document.