What is Thimble used for?

It is from Mozilla.org. It is free and has many cool features. It's Remix mode allows you easily modify existing Thimble projects to suit your requirements.

Thimble is a one-stop program bundling code editor, web server, web browser and developer tools.

It has an unbelievable number of cool features. Sure, I will test drive it!

• Make a change and see your changes in real time
• Has handy code snippets
• Has a built-in JavaScript console
• Add your files (drag and drop) and manage them
• Has both dark and light themes
• Embedded tutorials-follow or create OK
• Get code hints
• Edit CSS right in the HTML file
• Publish to Web
• Pick colors in the editor
• Test project on your mobile device
• Has a DOM inspector to work with HTML elements
• Has Image filters
• Take selfies

You start here.

Here is a video that explains it all:

What does the DETAILS tag do in HTML5?

Just as it says, when you click on Details tag (which appears as right-pointing triangle) in the displayed web page, the contents of the details are revealed.

Unfortunately it is not yet supported in Microsoft Edge or Mozilla Firefox although it is a html 5 standard. More on this here: http://hodentek.blogspot.com/2015/12/firefox-42-has-slight-edge-over.html and here.

Details_Edge

Details_Firefox

However it is supported on Chrome browsers version 12 and above. By the way it is also supported in the Intel XDK's emulators. However, when the code is run on the device, it is up to the browser of the device to render it or not.
The next picture shows how the tag would appear on a rendered page on an emulator in Intel XDK, as rendered as well as when the tag is clicked. This interactivity does not require any extra code.

However, Windows 10 Phone does not render the tag at all. It just displays the content and the iPhone6 does show the Details (but it is not interactive, probably a separate reason why it does not).