Compatibility
across various Browsers

HTML 3.2 seems to be the de facto standard for new browsers. But over 10% of the browsers in use do not measure up to this.

And there are some notorious discrepencies between Netscape and Internet Explorer.

There are many interesting features which you can find for your website. They will produce effects which are exciting, colourful, amusing, and grab the surfer to look further at your website. But unless the surfer is using exactly:

• the same browser,
• the same set-up for the same fonts and colours,
• with the same Plug-In options,
• using the same display size and resolution,

Sound and Movies
(Many users will not have implemented the plug-ins for these.)
If you are using multi-media you should warn the user, and also ensure that the electronically-deprived have something to look at instead.
Shockwave in tables can cause problems in NS2.

Images
(It is said that 60% of your visitors surf with the graphics turned off.)
Each image should have an ALT= clause to mark its place.
Users of Lynx browsers cannot handle graphics and if there is no ALT="" clause are presented with an ONLINE message.
Images should always have WIDTH= and HEIGHT= clauses to allow the space to be assigned while loading the remainder of the text to be displayed.
Mosaic and OS/2-Warp will not scale images.
<IMG BORDER=x> produces inconsistent results.

JAVA and JavaScript
Some browsers may not have either of these languages. Do not rely on them working.
Internet Explorer 2 does not handle Javascript, and IE3 handles Javascript in slightly different ways
Javascript 1.1 works only with NS3.
JAVA will not work if any of the images on that page do not have WIDTH= and HEIGHT=

Forms Submit using mailto:
The mailto: statement requires that the browser is properly set up to handle mail
Early versions of Internet Explorer do not have its own mailer and relies on invoking the surfers own mail package which is on his own server. It follows that the forms - mailto feature does not work on Explorer, and merely displays a blank mail form.
You can use CGI scripts on your own server to handle this consistently if you must have it.
The results transmitted from a form can be in a different order depending on the browser, so each item should have its own identification.

Frames
Several browsers will not handle frames. The heading frame has provision for a message or material to be displayed when accessed by browsers which cannot handle them.
Visitors who are using 640x480 displays may find the frames window rather small. Or even very small...

Tables
It is probably now safe to assume that all browsers will handle tables.
Empty table cells will probably be ignored; you can put in a space   character to avoid this.
<TABLE ALIGN="center"> may not work.
Border colours and table colours and backgrounds may not work in early versions of Netscape.

Lists
<UL TYPE=SHAPE> usually does not work

Blinking and Marquee
Netscape is alone in implementing BLINK, which was originally included for internal error messages.
Explorer is alone in handling Marquee moving text; other browsers will display an un-moving image.

<P> and </P> pairs
There is growing concern that the most recent browsers will not be as tolerant of <P> commands which are not closed with </P> We shall have to watch this one.

Fo more details on this try:
http://www.webstandards.org