You're off to a great start, and this game has potential. This may still be early in development, and you plan to add all of this, but based on what's there...
THE GOOD:
1. Music is excellent and very fitting for a sci-fi space shooter!
2. Artwork is good, and the enemy ships have a nice variety.
3. Attacks connect with the enemy, and explosions look great!
SUGGESTIONS:
1. Aside from the music... there are no sound effects. No shooting sound, no explosions, beeps, or chimes for when you acquire a 1-up or power-up. That would add a lot to the game. For example, you can find some free sounds on various websites: freesounds.org. Just check each sound's copyright to make sure you can use it.
2. It was pretty easy. Enemies didn't fire at you, they moved slowly, and they appeared one at a time. They should fire at you, and maybe you have them come out in groups/various formations... a horizontal or vertical line, box-shaped, U-shaped, etc. Make me mash my attack and throw more at me. Maybe even give some a shield you have to break through before you can kill it.
3. Some enemies appeared off-screen and were only partially visible. This made it frustrating when attempting to attack them. Since it was mouse only and a game played on my browser... if I went over too far to kill that enemy... if my mouse went off screen, my ship froze. Maybe try to keep all enemies fully on screen?
4. Level transitions felt clunky and quick. Maybe add some sort of text or a quick animation indicating you moved on to the next stage?
5. More power-ups would add a lot, as well. Spread shot, a bomb, increased speed, laser beam, etc.
OVERALL:
1. Overall Fun: It could be more challenging, and you could throw more at the player. Plus, more power-ups would go a really long way. That would add a bit more variety to the game.
2. Art/Graphics: This is the category I scored the highest. Very nice ship art, background of space and animations looked really good! Thumbs up here!
3. Controls/UI: Easy to control the ship, and it felt fluid. All attacks connected, and I was never confused with controls or what was happening.
4. Sound/Music: Great music but 0 sound effects.