Responsive web design is becoming pretty important in this day and age, and trying to ensure that it's implemented is something you'll need to consider when building browser games.
I have from experienced struggled to play games, as they are tiny when using certain resolutions, as someone who has terrible eyesight it's good to be able to zoom in, and have the website react and adapt to that.
It's also really good to reach a wider audience, if you can have your game scale to different sizes you're more likely to be able to allow people who are on tablets (for example) to be able to see and play your game!