IE11 CSS alternative to "unset"

You are looking for top: auto, it seems.

While top:auto will work in the case of top, it should be mentioned that auto is not simply an "IE equivalent" to unset or initial.

For example, the initial value for max-width is none (source). And the initial value for background-color is transparent (source). Each CSS property has an initial value, and for IE it must be explicitly set.

Initial values are listed on sites such as MDN, and w3schools (where they are called "default values").

With a question title like IE11 CSS alternative to “unset” many people will find this page from a search engine who have needs that differ from OP's. So I think an explanation of how to look up initial values is more valuable than a "just use this" type of answer.