Thank you for your answer.
I have no problem with the fact that Kendo charts with RGBA colors are wrongly displayed in browsers where RGBA is not supported however I would expect them to be displayed correctly in modern browsers (just like rounded corners), since I haven’t found anything in the documentation that suggests I shouldn’t use them.
“Any valid CSS color string will work here, including hex and rgb.” – about using colors in Kendo DataViz.
So I still think this is a bug, or the documentation is not accurate enough.
Anyways, this is not a problem for me anymore, since I’m satisfied with the charts’ appearance without borders, and I’m using different color schema depending on the browser’s type – RGBA colors in modern browsers and RGB/HEX colors in old browsers.
P.S. I have to use RGBA colors to make one bar highlighted (by fading the other bars’ color) in the bar chart. If you know any other way to reach the same effect without using RGBA colors (e.g. using series opacity in some way), please let me know.