
The question is not "can everyone see the site, regardless of system/browser?" Instead it is "how do I make my site great for my expected audience?"
The first question does not favor your expected audience. Everyone is rated equally, or more precisely, the edge cases are favored above anyone else.
To make a site work in pre-1997 browsers (or those browser who only support these technologies), your site might be visible in most desktop browsers. But:
The good thing is that:
But, the first question still focuses on the on those with the worst equipment (visually), basically leaving behind those with the latest equipment.
Look to the future, because that is where you'll spend he rest of your life.
The second question "how do I make my site great for my expected audience"?
This question does not care about the minority. It focuses on those that are important. It basically builds on the Pareto Principle - or better known as the 20/80 rule.
And...
So how can you change this unbalance? Well, it is definitely not by serving the edge-cases - that only expands the gab. Instead we need to find out who represents the important 20% - those who generate profit.
The best way is to know your audience, but it is a little different for websites than other types of media. If your website is a business card, introducing your company and products (like the majority of websites out there), then you cannot look at your existing clients. The people, who already do business with you, are the one group that no longer needs your online business card. Instead you need to look at your future audience.
Note: For sites selling products online, or includes web-services - not just static information - it is a very good idea to look at your existing clients.
That is best done by looking at demographics, age, online experience and log files.
Another thing is your log-files. If only 1% of your visitors use older-browsers, then it is much more likely that the important 20%-target group is using something else. Spending time "downgrading" your site, for the 1%-edge-cases, will widen the 80/20 rule.
On my personal site, my expected audience is fellow web-managers and web-marketing people - a group of people that I can comfortably rely to have the latest equipment. This is why my personal site is made using XML as the foundation, and XHTML/CSS as the visual desktop carrier. 1997-HTML is non-existent.
For some of the other sites I make - like Zociety Magazine, for young women - My target audience is not as advanced as for those visiting my personal site. So I have lowered the standard level to more general XHTML/CSS level, so that older browsers (Back to IE5.0) can display the site visually as well. But, 1997-HTML is still non-existent. Only 1% of my visitors need that technology (80/20 rule).
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