This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.
Element.runtimeStyle is a proprietary property similar to
HTMLElement.style, except its styles, that have higher precedence and modification. It does not modify the
style content attribute. It is available in old versions of Microsoft Internet Explorer.
Not a part of any specification.
Microsoft had a description on MSDN.
BCD tables only load in the browser