I provided a solution for the CSS.
I would say it is actually an error in the css provided by default, because that is messing up the normal inheritance of the inline styles of the table.

Sure it is a UX issue, and that is exactly the solution I provided in #10160.

Since this is technically not a defect in code, I'm going to convert this to a discussion.

That is not true. the file you are providing when content_css option is not given is loading this default css modules/oxide/src/less/skins/content/default/content.less

These lines here are breaking the inheritance of border style to the th and td elements.

e.g. consider this html:

<table style="border: 1px dashed red;">
<tr><td>something</td></tr>
</table>

normally the table and the cell will have border: 1px dashed red.
but the rule you provide

table[border]:not([border="0"]):not([style*="border-style"]) th,
table[border]:not([border="0"]):not([style*="border-style"]) td {
  border-style: solid;
}

Is making that the inherited border: 1px dashed red to be ignored.

The solution I provided is:

• reducing the specificity of that css rule
• adding more css rules to lookup for border styles in the table and applying it to the cells.