Helping to develop Red Hat's UI library called Patternfly. Its open source - check it out at Patternfly
For Accessibility, its important to build components using semantic html structures. For example, when building the Accordion component I used the dl, dt, dd structure over divs.
Every component has different states depending on the context. For example, the Clipboard Copy component I built as an expanded state where the information is displayed when the toggle is in focus.
Our pattern library using a modular approach. Which means components can be used with other components. When building the Context Selector I built it using the input and button components.
The chip component was an example of a component that I designed and built. Here, the chips are inside of the select component.
Definitely the hardest UI Development challenge was building and styling a Compound Expansion Table with tables within tables. Another challenge was making it work on responsive devices by turning the table styles into grid styles.