The HTML widget is pure static, means whatever html you feed it will take and display in the page.
The custom draw widget is very similar to html but it has added advantage. It has access to the data from the connection service.
So practically you can query your database, get the data and feed to the d3js library for building cool visualizations.
Using the awesome d3 library we built this clocks, analog clock displays that can be used on any dashboard page, one for each timezone.
Version 1 : Here is the d3 analog clock dashboard
Version 2 : Here is the d3 analog clock dashboard - part 2
The clock code is adapted from this great tutorial on d3 clock - ericbullington.com
Here is a quick video on how to add clock visualization to your dashboard