Pappy Fiddle: A big advantage of an online tool like the mandolintab.net one, or the one Michael Eskin has just created, is that the exact same program code will run on any device that will run a browser. That includes mobile phones and tablets such as iPads. To produce an installable program or app on a Windows desktop computer and both types of mobile device is a lot more work. And in the case of an iPhone or iPad it will cost the developer money for an Apple Developer licence, so it either won't be free or may contain adverts.

Though it's often not obvious to the user, there are two main types of online tool like these abc converters. Both load from a web server - so yes, they require a server to start with. The mandonlintab.net code actually runs on that server - the jargon is a 'server-side application'. The abc you type in gets sent to the server which sends its response for your browser to display.

Once loaded Michael's abctool runs entirely on your computer - the jargon is a 'client side' application. The abc you type is not sent to the web server, and all processing is done inside your browser.

And you're absolutely right about expressing your appreciation for free apps or programs that you use. Though that can be difficult to do; the feedback systems operated by Google and Apple on their stores, are not much good for that.

And I've just read what Reinhard wrote - and he's quite right - though it's not so easy on a mobile ;) (I've suggested to Michael that he uses service workers to operate offline, and I'm sure he'll get round to it when he's finished adding bells and whistles!)