How to configure CkEditor's styles dropdown? I have made changes to the configuration of CKEditor for the basic_html rule set as follows:



When I then try to use the editor, however, I get the following:



I have tried clearing the cache multiple times.
I have tried rebuilding the cache with drush cache-rebuild.

Is there something else you are suppose to do to be able to configure CKEditor in Drupal 8?
1. Your Custome style should be present in your CSS file. 2. Your FILTER SETTING should be proper. To check, same way as yours, I just enabled style and and put h1.heading-a|Title(which I took from css/base/elements.css for example purpose) in style dropdown and and it did show up as shown in the second image.


  


Second Image


  


Hence, the issue is, to properly put the Class|Key in the Styles dropdown section and make sure that it is available in CSS. 

Later to check custom one, I did put h2.myclass {color: red} in elements.css and saved and cleared cache. And as you can see it showed up in following image





UPDATE-1: FILTER SETTING

Now the reason why, Horizontal line and Underline iCon are not visible as you have stated in your comment. For that you need to check the FILTER SETTINGS. Have you played with Limit allow HTML tags and correct faulty HTML list which you can see at the bottom of the basic HTML setting page as shown in following image. If you delete &lt;hr&gt; from the list the icon will not shown up. Just I checked, if I remove &lt;hr&gt; form list the icon also gets disappeared.


  


And your Enable filter setting should be as shown in following image.