It seems like when modifying the CSS properties of elements when using JS methods (CSSStyleSheet, with insertRule or deleteRule, or CSSStyleRule.style.setProperty()), the underlying CSS has been updated, but the page itself not not reflect the changes.
The changes are only realised upon some reflow trigger, say after I've changed the attribute of the element, even if it's an invalid one.
May be related to this question: What are the conditions under which a browser will re-assess and reapply CSS selectors and styles?
What is the recommended way to modify CSS using JS? Apart from changing class strings, or a hacky way to insert and delete attributes?
Note: Testing on Edge / Chromium
After reading your post, I'm not sure if you are aware of these simple ones:
element.style = stylestring (IE: "display:block; color:red;")
or
element.setAttribute('style', stylestring)
These will be treated as inline-styles. So changed properties will take priority over the external css and the effect will be immediate.
Now, is it the recommended way? That's a tough one, I'll let others answer this question, I'm not really a W3C guy...