All you have to do is visit Paste HTML, enter some code, publish it, and look at what you made. Paste HTML supports HTML, CSS, and JavaScript. You can even use Markdown if you want.

Invisible HTML (left) gets picked up during copy-paste and has an extra line (right) ... Update, Jan 4th, 02:00 AM ET: Added another example of attack using invisible HTML/CSS.

Copy the code from elsewhere by selecting it, right-clicking it and clicking "Copy." Right-click where you want the code in your blog's HTML source and click "Paste" to insert the code.