The example that follows uses a Content Editor Web Part (CEWP) however, this practice can be used in any SharePoint Web Part that allows you to Edit HTML source using the icon in the Rich Text Editor

I was working with a client on a relatively small project where they asked that I come up with a project oversight and documentation solution that worked well but didn’t require all the overhead of their typical enterprise tools used to track tasks and accumulate and manage project documentation. The team never needed to directly access the SharePoint site, we worked entirely within the context of OneNote.