Problem:
the same CSS classes are used for styling of different widgets, hence when they nest, styles interfere.

Example:
MultiSelect in a Grid column filter. The filter form is placed in the column menu and menu item styling (.k-filter-menu .k-button) is propagated to all k-buttons in it, including MultiSelect's selected item tags that are also k-buttons and have styling with lower specificity.

Solution:
Use additional CSS classes for specific styling. That is .k-button is Ok for generic styling (color, border, hovers), and another, say .k-item-tag and .k-menu-item, for widget specific (margin, width etc.)