Matt's Script Tutorial Labratory
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Enter your HTML and JavaScript and whatever you want. Try viewing it. If it looks like you want use cut and paste to take it from the text box. Remember unless you place a body tag and give the test page some color it will be king of ugly. Warning: If you make a serious mistake in JavaScript it may shut off a webtv. I've done that to myself a few times (LOL). I know nothing about computers so this may not work with all browsers.
Also you can bring the scripts from my Library here to edit and test them before bringing them to your site.
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The short answer is no. Not on Webtv anyways. But you could store it in your e-mail for later use. I often work on things and don't have time to get into my page editor to save them. Now I can paste the contents into the small textarea and enter my e-mail address and send it to myself. I can then cut and paste it later. Don't worry about the size of what your pasting the window can hold entire webpages. Sometimes un-needed spaces may be added wich may cause you to need to make some mild corrections to what has been mailed.