Webbots, Spiders, and Screen Scrapers, 2nd Edition

Unpredictable Forms

You may not be able to tell exactly what the form requires by looking at the source HTML. There are three primary reasons for this: the use of JavaScript, the readability of machine generated HTML, and the presence of cookies.

JavaScript Can Change a Form Just Before Submission

Forms often use JavaScript to manipulate data before sending it to the form handler. These manipulations are usually the result of checking the validity of data entered into the form data field. Since these manipulations happen dynamically, it is nearly impossible to predict what will happen unless you actually run the JavaScript and see what it does—or unless you have a JavaScript parser in your head.

Form HTML Is Often Unreadable by Humans

You cannot expect ...