By default these are timed independently of Shotcut/MLT, and it will not give proper motion when exported or seek properly inside Shotcut preview. HTML/CSS provides an API called Web Animations to gain control of the timing, but that is not supported directly in our web runtime. Shotcut includes templates that integrate a Web Animations polyfill (builds the feature in JavaScript) with WebVfx. There is no way to expose this generically in our visual HTML editor. You can, however, build your own Shotcut filter and utilize MLT properties to communicate including with keyframes to the JavaScript. This is what Crop: Rectangle and Crop: Circle do.
If you want uncomplicated, you need to look to a tool like Tumult Hype. There are other Web animation tools such as Google Web Designer, but they make it difficult to integrate their runtime last time I checked a few years ago. Also, I may discontinue the inclusion of HTML into Shotcut since the technology we use is old, unsupported, and increasingly difficult to build.
I am looking to add another rich text technology from Qt that is still supported, and it has very good visual editor support. It is much more limited than HTML5/CSS/JavaScript/WebGL/Canvas but satisfies a lot of basic textual needs.