The HTML Designer allows you to visually design HTML files. To show it, just create a new HTML file or open an existing one. Notice the bottom tabs (Code | Design) to switch between HTML source code and visual designer.
All changes you make on the code, when switching to the designer, will be reflected and vice versa.
When the HTML file is open, the Tool Palette shows HTML elements (don't confuse these elements with PHP components), like HTML Button. You can add these elements to the code editor by double clicking or add them to the designer, by double clicking or dragging from the Tool Palette to the designer.
When the Code Editor window is active and an HTML document is open, the Structure Pane shows the structure of your HTML document. The Object Inspector shows the attributes for the selected HTML element in the HTML designer.