I can't talk about Unity because I don't know it, but AFAIK it currently doesn't generates pure HTML5 code. One needs a plugin to run it on browsers.
If you don't know / don't want to learn it, you have other options.
- its programming language is Lua. Lua is a very easy programming language. It resembles python (but it is easier). Also, lua is the fasted script language. Under the hood Gideros engine is written in C++.
You write your game once in Lua and export it to MS Windows, MacOS, Linux, Android, iPhone and to WEB.
Also Gideros allow minification, compression and encryption of both assets and source code. Its an extra layer of protection that will make reverse engendering harder.
I'm learning it since November 2019 and I'm loving it. I highly recommend it.
Openfl is a wonderful framework. You will code using Haxe programming language. Think of Haxe as ActionScript 3 on steroids.
Openfl exports to MS Windows, MacOS, Linux, Android, iPhone, WEB and consoles.
There are a lot of frameworks I didn't talked about. You can search for them on Google.
Now that I talked a bit about which tools to use, let's talk about some options on how to make money with game development.
1. Develop games as a freelancer. You can sign-up yourself in sites like upwork, and freelancer and take jobs to make custom games as well as convert games made in Flash to HTML5.
1.1 you must have a good experience making games in order to delivery what was requested. Otherwise you won't be paid.
1.2 you won't have any rights of games delivered.
1.3 Once a game is delivered you have to look for the next job.
2. You can sell exclusive licenses of your own games. You decide which game you want to do (based on your experience, on your personal taste, on what you think market is demanding, or even by asking directly to probably clients).
You have to get in touch with sites like Y8 and deal with them directly.
2.1 you may develop a game market is not interested in.
2.2 you have invest your time developing a game that may not be sold.
2.3 you have to invest your money buying assets (images, sounds...)
2.4 you have to digg for contacts of clients and sometimes its harder to get them. Sometimes they don't deal with an independent developer. They acquire their games directly from a partner.
2.5 it's a time consuming task to deal with many possible clients.
2.6 the process, from contacting a prospect until get contracts signed and receive the money may take months.
2.7 Your game must be well made, attractive and have a great playability in order to attract attention of your prospectors. The more amateur the game looks, the less it will be worth.
Even with all this drawbacks, it's one of the best way to make money.
3. You can sell non-exclusive licenses of your own game. It's very similar to selling exclusive licenses.
The differences are that in exclusive licenses the value paid by client is higher but you can sell the game just once and that in non-exclusive licenses, value paid is lower but you can the game many times.
Usually you would earn more selling non-exclusive licenses.
A very important think you must bear in mind if you want make a living selling non-exclusive licenses: NEVER, EVER sell non-exclusive licenses to game distribution services like famobi, game distribution, gamonetize, and others like them. They will distribute your game to all portals that use their services to get games and you will never be able to sell another non-exclusive licenses of that game. No one will pay for something they can have for free. You were warned!
3 - Monetize your games with ads. It basically works this way: you submit your game to a game distribution service like famobi, gamonetize and game distribution after adding their Ad SDKs.
Read what I wrote here:
I'm sure that there are other ways to monetize, but those are the ones on the top of my head.