overall well designed game. here is my feedback:
1) game took a long time to do a first time initialization. loading was stuck at 80-90% for over 1 minute. i don't know what graphic or sound caused that to load so slowly but i don't have a slow computer so that felt very odd to me like the game crashed and i wanted to refresh the browser but decided to wait to give you proper feedback. after the initial load and me winning the game. i did refresh the browser and the load time was minimal each time after.
2) the first time the user plays the game they get a tutorial of sorts explaining what to do and some level select options and music mute option. after i won the game and inserted the coin to play more i no longer find that screen and it automatically loads into the first level each time. couldn't seem to find a way to get a menu to show up to quit the game and go back to the title screen. i would like to be able to play specific levels at my choice. (EDIT: after writing this i finally found that space bar gets you back to the title screen and there is a menu button in the top right. the space bar is not intuitive of a button and i don't see it listed anywhere to press that to get back to the title screen. the menu was hidden because i was not in full screen mode and was scrolled down slightly to see the floor of the game. the overall size of the window caused some issues for me. maybe adjust the none full screen size down slightly so it all fits in a standard web browser at 1920 x 1080 size easily without needing to scroll around)
3) the game is very bright and when you hit the little blue aliens the screen get even brighter. consider some color options (users choice if possible) to allow for a darker theme to the game. users will experience eye fatigue over time playing games with this level of brightness and will be forced to walk away from the screen to avoid headaches and eye strain.
4) the tutorial / title screen instructions showed escaping from the mothership but i didn't see anything that looked like another ship as indicated in the picture. this image initially lead me to believe that there would be other ships chasing me during my escape but after playing it i realized that is not what it was trying to say. consider changing this graphic to show the players ship making its way to the arrow section of the level instead. this is especially confusing because the graphic shows the ship going left and the exit is always to the right.
5) hitting the blue guys and taking a -1s penalty felt faster than waiting for the ship to touch the floor. the blue mess that they leave behind should go away over time. those extra particles will eventually cause things to slow down for the player if the level is long enough or there are too many blue aliens in a specific level. I'm imagining you would be adding more levels at some point and could run into this issue.
6) The ship always takes off in the same angle and waiting for it to land on the ground is annoying. consider adding mechanics that will allow the user to hold the button the adjust the angle some so instead of taking off at a 45 degree angle each time it could vary somewhere between about 30-60 degrees depending on how long they hold the button. you could even make an indicator go back and forth in that range while they hold the button. also add something to make the ship go to the ground faster. right click or down button. anything to help make this time frame reduced would make the game dynamic much better. think of it like Tetris, without that ability to get the pieces down faster the game would take forever and not be nearly as fun.
i played 1 full time around then during the 2nd run through, i stopped on the 3rd level and decided i didn't have incentive enough to continue. i couldn't see my current fastest times (besides when i won the first game), i thought that there was no indicator for my current run time on the level and the level was static so once i find the quickest path, its always the same. i didn't find the way to get back to the title screen until after i already wrote all of this up so trying to find my fastest runs didn't feel possible and i don't think players will think space is the button you need to press to get back to the title screen to see them or select a specific level. i realize that all of this seems like stuff that is actually there and ultimately the problem is that the window that loads is larger than my screen so i couldn't see everything and the Fullscreen button is half way there. i can barely see the words since they are more than half way missing. i finally clicked it only after saying to myself, "there must be a full screen in the bottom right like the other games and maybe this little blue box is that."
the game is well done overall. i feel like if you make a few tweaks to it you will have more people wanting to play consistently. especially if you have a way for them to compete with one another somehow (overall scoreboard or 1 on 1 battles). if you update it please let me know so i can take a look at it again.