When you publish your article online you want it to be easy to read for your visitors. Chances are your website theme is already handling most of this, but here are a few simple things that make a big impact on the readability of your article.
Note: I am referring to article design specifically for the purpose of reader comfort. Design is definitely a whole different side of your website and your own ideas.
Increase the font size
You can probably increase the font size. The font size you use will depend on the font you choose. Don't intentionally make the font small to fit more on-screen. Let your readers relax with something they don't have to strain to read.
Adjust the line-height. That's the spacing between the lines in a paragraph. This will depend on the font size; give some space between lines — but not too much. Too tight and it will be hard for the reader's eye to follow the text down the article.
Too much and you'll have big space for the reader's eye to "jump" across from line-to-line (and your article will look strange).
Add some big margins above headings
Add some big margins above headings. This will help break up the article and give it some white space. This subconsciously tells the reader there is a change in thought coming next. But maybe more importantly it helps the reader skim your article quickly.
Pro tip: when your article is super easy to skim, make sure your headings are clear, intentional, and convey something actionable for your reader. Entice them to stop skimming to read.
Use bolding & highlighting to draw attention to key concepts
Bold and highlight the main thoughts in each section. This will draw the reader's eye, especially if they're just skimming your article. This may even stop their skim and hook them into reading it instead.
Don't overuse the holding and highlights, use them on key concepts you want the reader to pay attention to.
Use large headings for "skim-ability"
Make the heading nice and big. The same concept as the margin, a large heading is easy for the reader to skip to. Within reason, don't make your heading King Kong size that pushes all the other content around.
Those are baseline design ideas you should keep in mind for your website. Some of these are handled at the theme level; you set them and forget them. You don't need to worry about them for every article you publish.
And of course these are merely recommendations. Things I personally appreciate when reading articles online, and in turn use here to help you read through things more easily too.