[O] Ox-html: Replace <b> with <strong> and <i> with <em>

[Kaushal Modi]

Hello,

I am not an HTML expert. But recently off-list, I learnt that <b> and <i> tags aren't recommended to be used for styling any more (for a while now). 

Instead <strong> and <em> should be used respectively. 

If there are no objections, I can commit this little change to the master branch. 

References:

- https://developer.mozilla.org/en-US/docs/Web/HTML/Element/b

- https://developer.mozilla.org/en-US/docs/Web/HTML/Element/i#Usage_Notes

--
Kaushal Modi