1. If you use Microsoft Word, make sure you upload all of the image files that go with your image. Many users are confused when their images don't work in anything but Internet Explorer. That's because Microsoft Word saves images twice: once in a proprietary format other web browsers know nothing about, and a second time in standard formats like PNG, GIF and JPEG. If you don't upload all of the image files, which will be in a subdirectory called index_files if your page is called index.html (and so forth), then your images will be broken in anything except Internet Explorer.
3. Include plain-text links to your content and text alternatives (alt attributes) for your images that contain text or are crucial to understand the page. Otherwise browsers for the blind will not work. Equally important, search engines will not index your page properly. If people can't find you in search engines, they pretty much won't find you at all.
4. Don't build your whole site in Flash. While most people have Flash, browsers for the blind, search engines, handheld devices and users who do not like Flash will completely miss your site. You should use Flash for accent, pizzazz, and entertainment, not as your entire design.
5. CSS provides powerful ways to control the appearance of a page— but also leads to serious browser incompatibilities. You can limit these problems by using the HTML 4.01 Strict document type. When you design in "strict mode," Internet Explorer 6 and above will show behavior much closer to that of Firefox, Safari and other modern browsers. But only if you specify the strict DOCTYPE at the very top of your page:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
Otherwise, Internet Explorer runs in a "quirks mode" where it attempts to emulate the behavior of older browsers. Strict mode greatly improves cross-browser consistency of page layout even with Internet Explorer 6, and with Internet Explorer 7, compatibility is further improved although Microsoft unfortunately failed to implement the entire standard.
Strict mode does introduce some changes that may come as a surprise to those who have designed only for Internet Explorer in the past, without testing their pages in browsers like Firefox that aim to be standards-compliant even without a DOCTYPE. You can learn more about the differences by reading about quirks mode and strict mode on the excellent website www.quirksmode.org.
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