Here is some great advice from SitePoint.
Website owners should appreciate the following fundamentals and constraints of the web.
The web is not print!
Your site/app will not look identical everywhere. Each device has a different OS, browser, screen size, capabilities etc.
Functionality can differ
Your site can work for everyone but experiences and facilities will vary. Even something as basic as a date entry field can has a diverse range of possibilities but, ideally, the core application will remain operable.
Assess your project
Be realistic. Is this a content site, a simple app, a desktop-like application, a fast-action game etc. Establish a base level of browser compatibility, e.g. it must work on most two year-old browsers with a screen width of 600 pixels over a fast wi-fi connection.
Assess your audience
Do not rely on global browser statistics. Who are the primary users? Are they IT novices or highly technical? Is it individuals, small companies or government organisations? Do they sit at a desk or are they on the move? No application applies to everyone — concentrate on the core users first.
Examine the analytics of your existing system where possible but appreciate the underlying data. If your app doesn’t work in Opera Mini, you’re unlikely to have Opera Mini users. Have you blocked a significant proportion of your market?
Change happens
It’s amazing that a web page coded twenty years ago works today. It won’t necessarily be pretty or usable but browsers remain backward compatible. (Mostly. The <blink> tag can stay dead!) However, technology evolves. The more complex your site or application, the more likely it will require ongoing maintenance.
