Questions
Is my text uploaded?
No. It's converted inside this tab, and the page's security policy blocks it from sending data anywhere.
Is the HTML safe to publish?
With the clean option on, scripts, event handlers and javascript: links are removed. Turn it off only when you trust the Markdown and want the HTML exactly as written.
Which Markdown does it understand?
CommonMark, the standard, plus the GitHub extras: tables, strikethrough and task lists.
Why does HTML to Markdown change my layout?
Markdown has no colours, columns or classes. Headings, lists, links, images, code, quotes and simple tables carry over. Anything else is reduced to its text.
Why is an image missing from the preview?
The preview never loads images from other websites, so nothing leaves your device. It shows the image's description in brackets. The HTML you copy keeps the image.