Unable to add tabindex in anchor tag using RTE

Hi All,

I came to a scenario where I need to create an anchor tag without href attribute using RTE component. I am able to create anchor tag using HTML option in RTE, but the thing is that anchor tag is getting skipped by tab index and never focused. Then I tried adding attribute tabindex=0 so that that anchor tag get focus, but that tabindex=0 attribute is not getting persist in html. Any idea why it is not allowed and how I can fix this in RTE? Thank you in advance!

Example

I am adding below syntax in HTML of RTE  

<a tabindex=0 class="new-class">click here</a>

In DOM the HTML element I am seeing this

<a class="new-class">click here</a>

Expected rendition

    <a tabindex=0 class="new-class">click here</a>