When you spend your time building the game, distributing it and then promoting you will at some point consider earning money out of it. If it's not a side project to thearn the language, technology or tool, but a serious endeavour on the path to becoming an independent game developer able to make a living, then read on and see what your options are. The technology is mature enough, so now it's just about chosing the right approach.
Paid games
The first, obvious choice that may come to your mind might be selling the games the way it is done for huge AAA titles - with fixed, up front price. Even though the digital market is key and you don't need to print covers and put plastic boxes with your game in a physical store, to earn decent money on selling your games for a fixed price directly to the players you have to invest your time and money in marketing. Only the best games will get even or earn more than they cost, and you still need a lot of luck for that.
How much you charge for your game depends on the market, quality of your game and all the other small factors. An arcade iOS title can be sold for 0.99 USD, but a longer RPG style desktop game on Steam can cost 20 USD and both prices are ok. You have to follow the market and do your own research - learning by mistakes early on and fast is important.
In-app purchases
Instead of having people pay for your game even before checking it out first, you can offer a free game with an in-app purchases. In this case the game can be acquired without spending a dime - give the game to the players, but offer in-game currency, bonuses or benefits for real money that can unlock a hidden level, offer advantage over other players or refill the energy needed to play. Designing a good IAP system is an art on it's own.
Remember that you need at least thousands of downloads of your game as only a small fraction of players will actually pay for IAPs. How small? It vary, but assuming one person out of a thousand is not far from the average statistics. In this case the more people play your game the higher the chance someone will pay, so your earnings heavily depend on your promotion activities.
Freemium
With IAPs comes the freemium term - a freemium game can be acquired and played for free, but you can pay for extra (premium) features, virtual goods or other benefits. The word itself got negative connotation after big companies focused on creating games that the main purpose was to get as much money from the players as possible instead of delivering the fun part first. The worst cases were when you could use real money to pay for the advantage over other players, or restrict access to the next stages of the game unless you pay for it. The term "pay to win" was coined and this approach is disliked by many players and devs. If you want to implement IAPs try to add value to the game with something players will enjoy instead of taking it out and then charging for it.
Add-ons and DLCs
Add-ons or downloadable contents are a good way to provide extra value to the already released game, but remember that you'll have to offer a good enough and entertaining content to attract people to buy it. A totally new set of levels with new characters, weapons and story is a good material for DLC, but to have enough sales the game itself have to be popular, or else there won't be any players interested in spending their hard earned money.
Advertisements
Instead of actively selling the games you can also try to get yourself a passive income - adding adverts and relying on previous activities related to promoting your game may benefit, but your game have to be addicting, which isn't as easy as it sounds. You still need to plan it out, and at some point need to have a bit of luck too. If your game goes viral and people start sharing it, you can get a lot of downloads and money out of adverts.
There are many companies offering advert systems - you sign up and allow them to show adverts and get a percent out of the earnings. Google AdSense is said to be the most effective one, but it's not designed for games and it's pretty shady to use it for that purpose. Instead of risking of having your account closed and all the money blocked try to use the usual, gamedev targeted portals like LeadBolt. They offer easy to implement system of showing the adverts in your games and split the earnings with you.
Video ads are getting more and more popular, especially in a form of pre-roll - they are showed at the beginning of your game when it's still loading. And on the topic of where to put the advertisements in your game it really depends on you. It should be subtle to not annoy the players, but visible enough to make them click it. Adding adverts between game sessions on game over screen is a popular approach.
Licensing
There's an approach that can work as a monetization model on it's own, and that's selling the licenses. There are more and more portals interested in showing your games on their websites. They follow various strategies to earn money on your games, but you don't have to worry about all that as selling the license is usually a one-time deal. You get the money and they can get creative and work on their own to learn which way is the best one in their case.
Finding publishers might be hard at first - try to look for them at the HTML5 Gamedevs forums. If you're known enough they will reach you. Most of the deals is done through emails when talking to a dedicated person on the publisher side. Some websites have that information while some others are harder to get. When reaching a publisher try to be nice and straight to the point - they are busy people.
Exclusive license
The exclusive license is a type of license for one publisher - you've built a game and you're selling all the rights to it to a single entity along with the rights to redistribute it. You can't sell it again in any form while the publisher will be able to do so and earn money on it, that's why this exclusive deal is worth a few thousands of dollars. How much exactly? Depends on the quality of the game, it's genre, publisher, and many others, but usually it will be something between 2000 and 5000 USD. Once you've sold exclusive license you can forget about promoting that particular game as you won't earn more, so sell only if you're sure it's profitable enough.
Non-exclusive license
This approach is less strict - you can sell a license to the publisher, but you're not limited to the number. This is the most popular approach as with every new publisher (and they are constantly showing up) you can sell your games on the non-exclusive terms. Remember that with this license the publisher can't redistribute it further - it's often called a site-locked deal as they buy the right to publish the game on their own, given portal. The usual cost of a non-exclusive license is around 500 USD.
Subscription
There's also an option to get a passive, monthly revenue. Instead of getting a one-time payment you can get a small amount of money per game, per month - it can be something around 20-50 USD each. It's up to you if you want to get all the money up front or it's ok to have it sent throuout the time it is used. Remember that it can be cancelled, so it's not an indefinitely working solution.
Ad revenue
You can implement advertisements in your game on your own and try to find the traffic to earn a bit, but you can also do a revenue share deal with a publisher. He will take care of driving the traffic and will split the earnings - usually in a 70/30 or 50/50 deal, collected per month.
Remember that many new, low quality publishers will want to get your game for ad revenue instead of licensing because it will be cheaper for them and you might end up with earnings around 2 USD per game for the whole deal. Be careful when dealing with new publishers - sometimes it's better to lower the license cost for a known one rather than risking a fraud for more money with an unknown one.
Publishers taking your games for revenue share, and/or licensing may require implementing their own APIs, which could take extra work, so consider that in your rates too.
Branding
You can sell rights to use your game for branding, or do it yourself. In the first case it's almost like the non-exclusive licensing, but the client will usually buy rights for the code and implement their own graphics. In the second case it's like a freelance deal, but you're reusing the code and adding graphics provided by the client or doing them as they instruct you. You've got a game where a player tap the food? Change it to client's products and you've got yourself the easiest branding game possible. Prices in this model vary greatly depending on the brand, client, and amount of work you put in.
Other non-game focused
There are other ways you can earn money when building HTML5 games, and it doesn't even have to be game related.
Selling resources
If you're a graphic designer, you can sell your resources from the games you've created, or something brand new exclusively for that purpose at online shops like Envato Market. It's not much, but if you're a known developer it can be an extra passive stream of income.
Writing articles and tutorials
It is possible to write articles about your games and even get paid for that at the same time. Game promotion and monetization at the same time is a win-win, and if you don't abuse it the readers will also learn a thing or two. If you focus on sharing the knowledge first and use your games just as the examples it should be ok. Check out Tuts+ Game Development or similar websites for writing opportunities.
Merchandise
You can sell t-shirts, stickers or other gadgets - some devs make more money from the merchandise than from the games alone, but it only works on very popular and recognizable games like Angry Birds. Still, it could be another small stream of passive income. The more diversified your earnings are, the better for your business stability.
Donate
When all else fails you can try to put the donate button on your game's page and ask for support from the community. Sometimes it works, but only if the player knows you and feel that it will help you in your situation. That's why carefully managing your community is so important. It worked with the js13kGames competition where every participant got a free t-shirt anyway, but at some point could give back a few bucks to keep it going in the next years.
Summary
There are many ways to earn money - everything which apply to the "normal", AAA gaming can be, more or less, applied to casual HTML5 games. You might focus on selling the licenses, doing branding, or earning on a revenue share from the advertisements. It's totally up to you which path you're gonna follow.