What are some alternatives?
Rapidweaver - A WYSIWYG web editor for mac.
Proto.io - Create fully-interactive high-fidelity prototypes in minutes that look and work exactly like your app should. Sign up for a free 15-day trial and get started.
Sparkle - Dreaming a modern website? Dreading the HTML and CSS? Sparkle is what you're looking for!
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project
BLOCS - Visual web design app.
Zeplin - Collaboration app for UI designers & frontend developers