Device Independence

The hypertext you write is stored in HTML language, which does not contain information about the fonts and paragraph shapes and spacing which should be used for displaying the document.

This gives great advantages in that your document will be rendered successfully on whatever platform it is viewed, including a plain text terminal.

You should be aware that different clients do use different spacing and fonts. You should be careful to use the structuring elements such as headers and lists in the way in which they were intended. If you don't like the rendering on your particular client, don't try to fix it by using inappropriate elements, or trying for example to force extra spacing with empty elements. This may well end up being interpreted differently by other clients and looking very strange. You can in many cases configure the client displays each element.

For example:

• Always use heading levels in order, with one heading level 1 at the top of the document, and if necessary several level 2 headings, and then if necessary several level 3 headings under each level 2 heading. If you don't like the way heading level 2 is formatted, fix it on your client, don't just skip to heading level 3.
• Don't put extra spaces or blank lines into your text to pad it out, except in pre-formatted (PRE) sections.  
• Don't refer in your text to facets of particular browsers. Asking someone to "click here" won't make sense without a mouse, just as asking someone to "select a link by number" will betray the fact that you were using the line mode browser. Just leave a link. The instructions get boring as the user will normally know how to select a link.

A choice of senses

It is not even wise to assume that your readers will be using a screen-based browser at all.  The visually impaired, or those at work or driving, may be browsing the web using their ears rather than their eyes.  The "click here" makes even less sense for them.

A few obvious things to do are

• Use ALT attributes on images whose meaning is needed for the document;
• Don't use ALT attributes on purely decorative stuff;
• Use tags for the purposes for which they were intended, so that future browsers can do even cleverer things with them;

I can't give a complete summary of the do's and don't for making a web page "accessible".