What needs to be fixed

In IE(11), Fields of a Subform or Repeatable Form dont validate - Seemingly.
Truth is : IE tries to validate the invisible Subform-repeatable part of the subform - which ofc never contains values by its very nature - thus always producing errors

Why this should be fixed

Because Client side form validation is essential. The bug described just exists, because repeatable fields are relatively new and this issue hasn't been addressed yet.
Also the soloution is beautifully simple and implemented in a minute.

How would you fix it

Add the suggested line ( see below ) in to the File
/media/system/js/validate-uncompressed.js , circa line 129, right after
if(jQuery(fields[i]).hasClass('novalidate')) { continue; }
ADD the following line / code :
if(jQuery(fields[i]).parents().is("template") ) { continue; }

Explanation :
If a field is directly of indirectly encapsulated into a tag, it is part of the subform's/repeatable-field's template (/reserve) part, BUT NOT the actual form in use - and thus is NOT to be treated as the part of the form in actual use. Thus, if the validator sees the a given field has as direct or indirect/farther parent a element, the validator knows that this field is not part of the form and will skipp validation on this field. Viola.
Simple and beautiful solution.
( Reation of the validator is analogue to the line before, where it makes the validator skipp the field, it it sees a "novalidate" class attached ).

Ofc, the soloution must be transferred into the compressed version ( validate.js ) too.

Side Effects expected

None.