HTML style reformatting in letter head: is that a bug?

Hello everyone,

I’m facing an odd behaviour when editing letter head in HTML. Sometimes, the style attribute of the outermost div is rewritten when I save.

For example, the following code

<div style="margin-top: -15mm;">
  plop
</div>

is rewritten to

<div style="">
  plop
</div>

However, it’s not invariable. For example, the following code is left as is:

<div style="font-size: 0.8em;">
  plop
</div>

The rewriting also happens when combining the two. The following code

<div style="font-size: 0.8em; margin-top: -15mm;">
  plop
</div>

becomes

<div style="">
  plop
</div>

Note that the following is also rewritten.

<div style="margin-top: -15mm; font-size: 0.8em;">
  plop
</div>

My question is simple: Is that a wanted behaviour or is that a bug?

If that’s a wanted behaviour, could you explain it to me?
If that’s a bug, I will open an issue on GitHub.

Thanks a lot for your responses!