I don't know why I hadn't noticed this until my site had been up for 7 months, and most of you probably know, but there is a tag at the top called Doctype. The World Wide Web Consortium (W3C) has a set of rules fro websites to follow so that all browsers can display the content correctly. There is a validator at: http://validator.w3.org/ This tool will help you validate your html. I found that many sites on the net are technically invalid. I have noticed that even yahoo fails. At the same time, yahoo displays incorrectly on my aunt's imac. The site also validates CSS files. Here's the SEO/webmaster part. If the html is valid, you know that it can be easily read by spiders. Also you know that any browser can read your page and view it correctly. BTW I searched to see if this had been addressed, but found nothing.
It is always good to have you pages validate. All my recent designs are XHTML 1.0 trans or strict. Initially I found it a pain switch over to this new style of web design. By this I mean complex CSS, less tables, more divs etc. But in the end I believe it is for the best. Many of my sites I designed a long time ago (like some sites you mentioned) don't validate for whatever reasons. Most of the time its for little reasons. The arguement has been on these forums before, some people claim valid sites have a slight advantage while others say it makes no difference. Neither side really has any proof. I would say get in the habit of the latest web design techniques and methods either way. Validation can only help Cheers
Great tool. I tend to forget to use it, but get a reminder every once in a while when someone points out that unless my site validates 100% they won't bother signing up for my sites. I understand that there are positives in being standards compliant. Not really sure why some people are so anal about it though.
Do bear in mind that several components in perfectly valid HTML will fail the W3C validation... W3C is a set of recommendations rather than standards and they have targeted certain HTML elements as "deprecated" -- that doesn't mean you cannot or should not use those elements but they may cause your page to "fail" W3C validation and that in itself doesn't matter at all. Anyone who "refuses to sign up at your website because you are not 100% W3C compliant" doesn't know what s/he is talking about and probably isn't much of a loss anyway. Also bear in mind that the DOCTYPE declaration is supposed to instruct browsers as to which set of W3C recommendations/"standards" your page is following, so putting an incorrect DOCTYPE declaration at the top of your page won't help.
What browser can't interpret "<center>"? I'm not disagreeing with that. Just pointing out that I see frequent posts to the effect of "why does W3C say this code is invalid?" and I'm pointing out that just because W3C has recommended that something be deprecated doesn't make it so... take a look at how old some of those recommendations are and you'll see what I mean.
IE FireFox Opera Those 3 browsers are pretty much all you have to worry about. Have a copy of each installed and make sure your site looks good in all of them. It is not that hard to do.
You're right. Usually I just tell them not to worry as the site works fine. If they don't want to use my service. Fine. Sometimes I'll actually send them on to a "competitor".
I would add Safari to this list. It doesn't render pages the same as Firefox or Mozilla. You don't have to have a mac, just check out some of the free Safari screenshot websites.