<sub> tag is ignored in Slide mode

### Is this a hackmd.io issue?

- [x] Yes, the bug happens on https://hackmd.io
- [ ] No. If it's a self-hosted CodiMD instance, please fire up the issue in the repo of CodiMD: https://github.com/hackmdio/codimd

### What's the problem
I want to reduce some text size, in order to make room for an image. I use the `<sub>` tag, which is correctly rendered in Preview mode. However, in Slide Mode, the tag is ignored, and [the text is so large that the image gets pushed outside the screen](https://hackmd.io/p/SkR7GhwqN#/)
#### Current behaviour</sub>

> Screenshot

Note with reproducible content:

https://hackmd.io/p/SkR7GhwqN#/

#### Expected behaviour

The text, which is correctly shown in a small font in the preview area, should be rendered in a smaller font, in Slide mode too.
#### Environment

I use HackMD on:

**Desktop**
- macOS
- Browser: Chrome
- Browser Version: 73.0.3683.103