Hub is starting to implement more strict sanitizers. The `<font>` tag you have is no longer supported/valid HTML according to HTML5.
Esri as a whole is considering this more strict rule set for all applications for security reasons...Hub has begun implementing the more secure rule set ahead of the ArcGIS Online item details page. Other applications within Esri are also updating to be more secure so I suggest you make edits to remove the legacy HTML and stay current.
If you're looking for a way to get larger sections, I suggest you style to your elements or make use of the various heading levels available in HTML....such as `<h3>`
If you're looking for ways to bold, I suggest using `<strong>` or another valid HTML formatting tag.
Happy to answer more questions or concerns if you have them.