Understanding your audience is one of the trickiest parts of HTML5 and CSS3 web planning. You need to anticipate the audience in a number of ways in order for your site to be functional.
Determine whom you want to reach
Before you make a lot of design decisions, you need to think carefully about the type of person you're trying to reach with the website.
Try to anticipate the mindset that people have when they use a particular site. For example, a student simultaneously worked on two sites: one for a graduate program at a university and another for a spa and salon. She had to think quite differently about the users of the two sites, which had implications for how she approached each step of the process.
The graduate program page was part of a website for a university. The university already had its own style and branding guidelines, official colors, and a number of (evolving) standards. The potential users of this site were graduate students seeking online degrees. The focus of this site was all business.
People were there to learn about the graduate program and set up their schedules. They wanted information about classes, instructors, and schedules, but they didn't want anything that interfered with the problem at hand. The writing was efficient and official, the color scheme was standard, and the layout was also official.
The spa and salon page had an entirely different feel. The owner loves design and spent long hours picking exactly the right paint color for the walls in the physical space. She's really happy with her brochure, and although she's not sure exactly what she wants, she knows when something isn't right.
She wants to give her customers information about the salon, but more importantly, she wants them to get a sense of how invigorating, relaxing, and feminine the experience of visiting her salon can be.
These two sites, although they require the same general technical skills, demand vastly different visual and technical designs because the clients and their users are vastly different.
Of course, someone could simultaneously be a graduate student and a patron of the salon, but the person would still have a different identity in these different sites. If you're going to a university site, in a student mindset, you want quick, reliable information. If, after you sign up for classes, you're looking for a salon, you likely want to be pampered.
Websites are experiences. The design of the site should reflect the experience you're trying to give the user when he visits your site.
Find out the user's technical expertise
Understanding the user isn't just an exercise in psychology. You also need to estimate the users’ technical proficiency because it can have a major impact on your site. Consider these issues for the typical user:
Whether the user has broadband access: University students, hard-core gamers, and web developers often have high-speed Internet access, so they don't mind a page with lots of video, multimedia assets, and large file sizes. Lots of people still use dialup connections or have limited bandwidth. If your audience has slower connections, every image creates a delay. Audio and video assets are unavailable to this group.
Whether the user has a recent browser: You have no way to predict which browser a user has, but think about whether your target audience has a reason to install any of the current browsers. If most people in your audience are still using ancient browsers, using advanced CSS and JavaScript tricks on your page may not be the best choice.
Whether the user has a recent computer: Technical people tend to assume that everyone else keeps up-to-date on technology. That's not necessarily an accurate assumption.
Whether the user has certain proficiencies: If you include a Flash animation, for example, the user might not have the right version of Flash installed. You have to decide whether it's reasonable to expect the user to install a plug-in.
Whether this will be a largely mobile application: These days, every website should be considered a potential mobile site, but if a large percentage of your visitors will be using mobile devices to view your page, this will have implications on your design.
This process isn't about stereotyping, but you must consider the user while you're building a site. You want to match users’ expectations and capabilities, if possible.
Of course, you're making assumptions here, and you may well be wrong. Be willing to adjust your expectations after you meet real users. (For professional work, you must meet and watch real users use your site.)