On the latest version of the tests-passed branch, the iframe embeds without issues. Note that the style and title attributes that you have set will be stripped by Discourse. You can set width and height attributes though. For example:
<iframe src="https://www.tickcounter.com/widget/countdown/5847336" width="100%"></iframe>
What do you see when you navigate to the post and then open the “elements” tab of your browser’s developer tools?
How about if you open the “console” tab on the developer tools? Are there any errors?