Do you remember the famous dictum of Marshall McLuhan, " The medium is the message "? It is particularly apt in writing for the World Wide Web because the likelihood of people getting the message you're putting out depends on how easily it is made available to their searching on the internet. This depends on writing script that contains the actual key words or phrases that a potential viewer might use to compose a search query string at a search engine site or in their search through a directory's categories looking for a site such as yours. Having carefully chosen the keywords, the writer should implement them as frequently as naturally possible. Over-use is deemed spamming and will result in your site not being considered for inclusion [ about 5% frequency in the body of the text is the recommendation - headings and titles chock full].
There is a hierarchy of weighting attached to the positioning of your presentation as it is seen by computers and directory editors. First in appearance and importance is the title and description of your site that you place in the header of your document. Then comes any significant words that are in the name of your domain. Some value may be given to a list of key words used in your text if you care to compile it and place it in the header, but they will be found anyway. Then there is a sequential ordering of valuation placed starting at the top of each page (beginning with the home page) and going down through the order of title and subtitle, heading and subheading and the accompanying substantive text. What you would expect. There is a framework built in to the document that can be used to emphasize this for the computer's recognition.
"Theme development" is a catch word these days for keeping an overall focus on the subject of the site and its relationship with similarly oriented sites. Certainly it would be good to make every word count considering the nature of the media and milieu. Web surfers don't have much patience. They want to get right to the matter as efficiently as possible. There's so much out there to consider!
Another related maxim is to " write above the fold " ; a newspaper slogan that here refers to making the essentials of the whole site available to the viewer in the initial window when it opens, without scrolling down. Ancillary to this is how well you manage to break up and make available all the rest of your content. Navigation, pagination, sectioning; all come into play here more intensly than the relaxed and expansive atmosphere of book reading.
That about roughly covers it in a generalized way, touching on the factors peculiar to web functioning, documentation and the physical and emotional human aspects of web surfing activities. Using these disciplines in writing will open up great and unique opportunities to communicate in an interactive way with a wide audience that is targeted as having a special interest in the subject of your site.