When creating any GUI, mobile or web UI, one should keep usability in mind. Good starting points are your target platform's human interface guidelines (HIGs). Here are some examples for well written HIGs:
- KDE Human Interface Guidelines
- GNOME Human Interface Guidelines
- User Interface Principles (Microsoft)
- Human Interface Guidelines (Apple)
- Design for Android
- Material Design
A good starting point for accessibility is the W3C's Web Accessibility Initiative (WAI). They published the Accessible Rich Internet Applications Suite (ARIA). WAI-ARIA provide a series of valuable documents that are available in the WAI-ARIA Overview.
For creating wireframes, Pencil is worth considering. It is open source, uses gzipped XML files and is simple, yet powerful (but has some rough edges). Pencil is also available as portable application
A possible alternative is Inkscape, which is an open source vector graphics editor using SVG. It has a steep learning curve, but is very powerful.
Proprietary tools include Figma and Balsamiq - both offering SaaS solutions.