There are many places where you can practice writing online. There is of course here, WriterShelf. I like it the best because of the ability to have multiple pen names. This can be helpful if you're writing on multiple unrelated topics.
Still, it's good to have different places to write in addition to WriterShelf.
Dreamwidth.org is another good place to practice writing. It's set up like a diary, where you make chronological entries. It's been around for awhile, and as a result, the design is definitely somewhat dated. You can certainly change the theme around a bit, but no matter what you do you won't really get a modern look. Still though, you're there to practice writing, and as a writing venue, it works just fine.
Speaking of diaries, why not just go with a diary? my-diary.org allows you to create public or private diaries.
If it's simply a really good editor experience you're seeking, and not the ability to publish the writing, then you'll like Calmly Writer. It's a distraction-free writing spot. You can customize it if you choose, including optional sound effects. I tried out the Typewriter sound, which was fun. Basically what you see on Calmly Writer is a blank screen in which you can write, and a discreet menu button, nothing more. The menu button is where you can customize the editor. As mentioned, you can't publish your work there, but one of the options on the menu is to save your work to your computer or to your Google Drive. If saving to your computer, you have the option for several different formats: .txt, .md, .htm, and .docx. The editor also allows you to insert images, although again this is done through the menu. Calmly Writer is good if you need to "just write", but it won't be the best choice for those putting in a lot of links or images, since that requires constantly using the menu button.
If you want your writing to be private by default, then you should give Penzu a try. Unlike most writing sites, Penzu is completely private so you don't have to change the settings for each entry to make it private.
A good option for a semi-private diary is Diariste. It's a community of diarists. You can choose whether your writing is seen only by you, or by the other Diariste members.
If instead of writing for yourself, you want the world to hear you, Medium is a great option for writing. It's ideally suited to informative articles through personal experience. It's also good if you're wanting to try out article-style writing with keyword research, since the design format appears journalistic in style.