In Github semuxproject:
Light client
https://semux.online/
Is this official service?
This is developed independently by a Semux user. If you need help, or more information, ask witoldsz on discord, he is the person who created it.
online wallet are always very risky of being hacking so better to use offline local official wallet , i don't trust online wallet even created by the developer of the project .
Be safe . Thanks
Well, if the website has a javascript client side wallet and it open source then i'd say it should be safe to use.