If you are serious about founding your startup, learning HTML and CSS first should be high on your “Founder literacy” agenda. They are both invaluable skills and both are very easily to learn. You can get up to speed in a couple of hours!
Front end skills such as HTML and CSS are a must before you even think about learning to code. There are many free resources out there. Many of them are either fragmented, unsystematic, too dull to follow or takes too much perseverance to finish. Here’s a great free course on Udemy by Zenva that I highly recommend. The course is very hands on and only takes a couple of hours. You could go through it all in one sitting though I wouldn’t recommend it. A better pace would be to take 30 minutes a day over a few days.