A 'webpage' which is only displayed well in some specific browser (did someone say IE?) and not in others, is not a webpage. Please make real webpages. Stick to the standards. A label "best viewed with ..." is not a quality label, on the contrary. If you really want to use some fancy feature which is browser-specific, it should be something cosmetic which will not prevent people with an other browser from viewing the essential content of your site. Why would you want to create webpages which can't be viewed by a lot of people anyway? If that's your goal, then you'd be better off making no webpage at all.
In the good ol' times, one could simply use <A href="#top"> instead of making a named object "Top" at the start of a document. When clicking this link, the browser returned to the top of the page. Unfortunately this doesn't seem to work anymore in most newer browsers, so I recommend to always create a named object.
Always make sure to specify your images' width and height (these are inserted automatically if you insert an image with the "Image" button), it will speed up the displaying of your page in browsers and maintain lay-out if the image can't be loaded.
You can use an animated GIF as background, but it will only be displayed in newer browsers (i.e. almost all current browsers, at the time of writing this). In older browsers, it won't be displayed at all. (You can use a JavaScript which chooses between an animated background and a still image according to the type of browser).
If you have a page which is used within frames, and which contains a lot of links to external pages (other sites) which must replace the entire frame set, it's easier and more efficient to use a <BASE target="_top"> tag in the file's header than target="_top" in each link. Of course, you need to use target="_self" for all the local links if you do this, or they'll replace the frames as well!
When creating a site with frames, make sure to provide links to the most important pages (like the navigation menu) in the NOFRAMES section of the frameset. Otherwise search engines may not crawl your site, and people with frame-incapable browsers won't be able to browse your site. If you think there are almost no such people anymore today, read the section about accessibility.
When you have inserted an aligned image (e.g. with attribute align="left"), and you want to start a new line below this image, include the attribute clear="left" in the br-tag. This will start a new line below the image, even if there is still room for text next to it. Use clear="right" for right-aligned images, and clear="all" for both.
Be very careful with displaying your e-mail address on your pages. One of the most irritating things on the Internet today, is so-called "SPAM", which stands for unsolicited publicitary e-mail. There actually are people who do nothing else than searching the Internet for e-mail addresses to add to their lists. Once you're on such a list, you're bound to receive massive amounts of crap in your mailbox every day. There are even automatic programs which "crawl" through webpages to find e-mail addresses. A way to prevent your address from being found by such people or such programs, is to only provide http forms on your pages, and no direct mailto: links. If you can use a cgi script (like cgiemail) to send the form to your address without having to type it in the webpage itself, you're 100% safe. Otherwise you can still hide your address by coding it with a JavaScript which decodes it at the moment of sending the form.
If you do receive SPAM, I can simply tell you that your address is lost. There is no way to stop it once it has started, except by discarding the e-mail account.