Standard compliance, especially in something like a website builder should be a top priority. If the builder doesn't work in major browsers, why there be any confidence that the sites it produces will?
So, you are saying that there is no plan for it to work outside of Chrom(ium) browsers? It doesn't solve the problem of forcing someone to use a specific browser to use the platform.
It does not inspire confidence in an end product will work everywhere and be of high quality when the product to create it is buggy and limited.
Standard compliance, especially in something like a website builder should be a top priority. If the builder doesn't work in major browsers, why there be any confidence that the sites it produces will?