Looking for a word processing software where it’s much easier to set up your styles. There are several modes.
Design mode let you set style properties. You can change font size with slider, edit html code, edit style name. You have many ways of selecting colors. You can set style shortcuts. You can save design to use on other documents.
Content mode ignores what paper you write on. It is only intended to show the content. This mode has little layout to remove distractions. You can however easily move between other documents.
When you are finished writing, you can adjust the content so that it flows correctly. Don’t know if it make sense to have it’s own mode for this.
I use Cherry Tree (FOSS) in desktop and online the Blue Velvet Editor (it’s not OpenSource, but maybe good for the searched inspiration). Its a free complete online editor which stores the files locally in html format. So you can open it in the browser and also use it for easily create a webpage if you have a host. (the editor is part of the SSuite, works also in mobile, no registration, excellent privacy and TOS).