Save as HTML (legacy) > Clean, simple HTML best practices

I have some publications that are very nicely formatted in InDesign to produce great PDF ebooks. However, I'd like to also use the same .indd files to produce some very basic HTML versions, with no fancy fonts or formatting: something suitable for viewing on a cellphone, for example: responsive and with the graphics thumbnailed. After playing around with this just a bit, by using the Export > HTML5 and Export > HTML (legacy) I'm thinking:

a) I probably need to export not as HTML5 but as "legacy"

b) I probably need to create a CSS file that assigns the many paragraph styles in the InDesign document to some basic HTML formatting.

c) I would like professional advice from someone who has tried this before so that I may learn from the mistakes of others rather than repeating them myself.

Any advice?