2.1.8 [W3C-TTML-1.0] Section 10.2.3, dur
The specification states:
The semantics of the dur attribute are those defined by [SMIL 2.1], § 10.4.1, while taking into account any overriding semantics defined by this specification. Note: In the context of the subset of [SMIL 2.1] semantics supported by this specification, the active duration of an element that specifies both end and dur attributes is equal to the lesser of the value of the dur attribute and the difference between the value of theend attribute and the element's begin time.
IE10 Mode and IE11 Mode (All Versions)
If both dur and end attributes are defined, then the dur attribute is ignored.