Cause: this is because muted works similar to value for <input>: if it is present inline when the element is first rendered, it sets the initial value; but if it's not, just changing the attribute has no effect, we need to set the corresponding property.

It's a bit frustrating that there is no centralized documentation on the correspondence of HTML attributes/properties... there may be other attributes like this, and if there are we need to add them to the list too.

@yyx990803
open https://www.w3.org/TR/html5/single-page.html and drop these code in console:

[...new Set(document.body.innerText.match(/default[A-Z]\w+/g))]

// ["defaultView", "defaultPlaybackRate", "defaultMuted", "defaultChecked", "defaultValue", "defaultSelected"]

view belongs to document and playbackRate does not have a corresponding attribute yet, so the current list is complete.

Looks like playbackRate doesn't have a content attribute though. It's property only.