In the last couple of years HTML5 have turned from a trend to standard widely accepted by web designers and web developers. Though nearly as the same time a new trend have appeared - coding/design in the browser. Many HTML5 tools were developed and recently some very flexible HTML5 WYSIWYG editors became favorite to many professionals and non-professionals.
Probably there is no need to explain why HTML5 is so popular, as there are so many good reasons, mostly related to ease of use as well as extended HTML5 functionality.
Our team committed extended research through social media regarding latest HTML5 WYSIWYG editors that are favored by web developers. Here are the top 3 free HTML5 editors that received the most votes.
Brackets

Brackets is editor for HTML, CSS and JavaScript, more interesting fact is that Brackets is built in HTML, CSS and JavaScript. And due to fact that this editor is open source, being familiar with HTML, CSS and JavaScript may give you the option to fork it and make it suitable for your work.
Mainly useful for front end designers and developers. Editor is well optimized for HTML, CSS, JS as well as derivates like CSS preprocessors - Sass, Less, etc...
Some of the most notable features are - Live Preview, Quick Edit, Quick CSS edit/declaration, Color Selector, Bezier Curves, JSLint, etc...
The list of features is very long, as well as there are many extension that can increase the number of features.
In general Brackets is extended version of Chrome's Developer tools. Other language support is not that good, but without a doubt this is probably the best HTML, CSS and JS editor.
BlueGriffon

It is based on Gecko - the rendering engine of Firefox. BlueGriffon comply with the W3C's standards like HTML 4, XHTML 1.0, HTML 5 including XHTML 5 plus CSS 2.1 and parts of CSS3.
Interface lookalike popular office text editor, very intuitive and suitable for beginners. Preview in browser is quite easy and actually in Linux few predefined browsers can be accessed easily for cross browser testing.
One click WYSIWYG editor include all popular HTML elements, BlueGriffon even offer powerful SVG editor.
What makes it really powerful is the number of free and paid add-ons and some definitely worth mentioning - FireFTP, Mobile Viewer, Project Manager, etc...
BlueFish

All these applications deserve detailed reviews, so many features, capabilities and without a doubt money and time savers.
There are many other free editors that came up in our research and for sure worth mentioning, like CoffeeCup and Aloha Editor.
What is your favorite free HTML5 WYSYWIG editor?