I think you'll struggle by starting off with those requirements.
- Step-by-step debugging: More commonly, you're going to want to do this in the browser itself rather than in the IDE. If you have a bug that only happens in Chrome 6 then being able to debug JavaScript in an IDE isn't going to help you much
- Visual UI design: Again the issue is that, in the end, what it looks like visually in your IDE is unimportant, it's what it looks like in different browsers, on mobile phones etc. that matters
Probably the closest thing to what you're after is Dreamweaver, or if you're more comfortable with MS products you could try Expression Web (never used it myself). Personally I use Komodo Edit, the latest version has support for HTML5 and CSS3, and there's a paid version which adds a number of features, but neither are IDEs like you're (apparently) expecting.
In response to your edit: Komodo Edit has built in support for jQuery (ie. it offers code completion and tooltips), and you'll probably get a long way with a CSS framework (or even an HTML5 one) for ironing out the browser differences, but primarily front end web development is about editing text files and looking at them in browsers. The sort of things that IDEs are generally helpful for are not really major factors.
Further edit: Also, if you want a JavaScript widget framework, you probably should look at one of: