CSS Warning : " Expected color but found ‘null’/'none' " on Firefox

[InakiUrrutia]

Version

5.1.2

Reproduction link

https://echarts.apache.org/examples/en/editor.html?c=line-stack

Steps to reproduce

1- Open Firefox, Go to Echarts examples, find Stacked Line Chart.
2- Open web developper console.
3- Go to console Tab and then click on the 'CSS' filter to display CSS logs.
4- On the graph, over the graph to display the tooltip, the warnings should appaears on the console.

What is expected?

No warnings

What is actually happening?

CSS Warnings apparearing whenever i move the mouse with tooltip open:

Expected color but found ‘none’.
Expected color but found ‘null’.

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[plainheart]

It seems this warning information will still be thrown as long as the mouse hovers on the line series, even if no tooltip is specified.

[plainheart]

Maybe it is caused by the invalid value none/null for canvas context.
This is just in the warning level, so I'm not sure if we need to fix it.
Related source code: https://github.com/ecomfe/zrender/blob/master/src/canvas/graphic.ts#L456-L470
@pissang Please take a look.