[Feature] WCAG 2.1 AA compliance

### What problem does this feature solve?

We want to use E-Charts library in our SAS web applications, for our products WCAG 2.1 AA is standard for a11y. As per your official website, we see following statement for a11y.

"When thinking about a visualization, we naturally associate it with what you see, but there are other dimensions to viewing. The W3C has developed the Accessible Rich Internet Applications Suite (WAI-ARIA), which aims to make web content and web applications accessible to more people with disabilities.

ECharts 4.0 complies with this specification and supports automatic generation of descriptions based on chart configuration items, enabling people with visual disabilities to understand the chart content with screenreaders, so that charts are accessible to all."

Does that imply, the library is covering WCAG 2.1 AA standard ? If not, are there any workarounds or plans to implement this ?

### What does the proposed API look like?

NA