I am looking for recommendations for a starting website to learn how to write HTML code
This question seems a bit weird... what do you mean by "sandbox"?
Usually you simply practice writing HTML by using a text editor and opening the local file from the browser.
start here at w3schools.com. They provide a niftly little sandbox with sample code for all your web design element questions.
Notepad + any broswer - This works well for me. Just save your file to .htm
Or if you want, get FireFox or Opera, go to any site (say, stackoverflow.com or w3schools.com), view the source, edit away and then apply the changes. Don't worry, the changes only affect a single tab and doesn't changes anything on the web.
Sandbox for HTML? you must be kidding.There are no chances of getting hurt even if your HTML goes wrong. So you don't need a sandbox.
Use any decent editor which gives a two-tab view for Source-code and Quick-view, and you are done. You can use MS Frontpage or EditPlus, both offer these features. You don't need to save to see the effect.
Please don't clog the bandwidth for just testing and debugging HTML. It ain't worth it.
Some things don't work with Javascript when served from file:// due to security protocols, and sometimes it can be too much of a pain trying to get a webhost up and running for experimenting with stuff.
I have found to be a convenient playground tool, with the benefit when you mangle something up and you want help to work out what you did wrong you can post the link to somebody and they can see what you've done without you needing to worry about security, hosting, or firewalls.