I first noticed this bug on chrome canary and now occurs with the latest update to chrome beta. I have logged it as a bug to chrome, but wanted to get some additional input if this bug is indeed chrome related or is actually magento.
Chrome Version: 55.0.2883.28 beta (64-bit) on Windows 10
If I remove the float on .admin__legend, form changes to this
It seems that the legend tag's width is unable to be changed, which may be causing the issue and the additional white space inside the legend tag.
Here is what I have tried
Ultimately I could solve this with position: absolute; but I feel like that is cheating.
Let me know if you think this is a chrome related issue or something magento needs to prepare to fix. Also I'd be interested if anyone can come up with a simple fix.