This article shows the steps to add the Content Editor web part to SharePoint online site.

SharePoint-2013-Hosting

OfficeUI Grid

Content editor web part is used to display your rich content and custom functionality.
Prerequisites create a trail Office 365 E3 license account.
Just follow the below steps,
Step 1
Go to the SharePoint admin center –> Active Sites, click create,
Step 2
Select “Other Options”

Step 3
Choose More Template from the dropdown
Step 4
Fill in the Title, web site address and add your admin name if your the admin add your name
Step 5
Now go to site contents –> Site Pages –> New –> Web part page
Step 6
Give your page a name, select the layout and destination as site pages
Step 7
Add Content Editor Web part
Step 8
Add your HTML file from Site Assets to the web part
Once added click “Stop Editing”, your page is ready!!!