I won't say which sounds more natural, since neither sounds more natural than the other. Instead I will tell you the slight difference in meaning.
A– implies that the idea to create a navigation menu came to you while you were practicing HTML.
B– is just a factual narration of events, with no importance given to any one event.
If what's important is that you created a navigation menu, I would say it like this: "Today while practicing HTML, I decided to create a navigation menu."
In BOTH cases, it is preferable not to repeat "I". You only need it once. More is redundant, it sounds a bit childish:
"Today I was practicing HTML and decided to create a navigation menu."
"Today I practiced HTML and decided to create a navigation menu."
· 4 weeks ago