HTML is the computer language used for writing and formatting pages on the Internet and what you see when you view a web page is your browser's interpretation of HTML.
On Google Sites you can edit your page content source by clicking Edit page and then the HTML button on the edit toolbar. However, Google Sites does not allow full HTML modification — some codes are not permitted and you have to make gadgets for them.
The following tutorials are good starting points to learn HTML:
Click Edit page ► Insert ► More gadgets to view and search thousands of existing gadgets in the Google Sites gadget directory.
Most users find Google Drive more convenient than other Google-based hosting options such as Google App Engine.
Store your gadget and all its required files, share and collaborate, manage file revisions, and benefit from other features of Google Drive.
Note: Before making a gadget you might want to use Dynamic Content Writer to test and customize your code and monitor changes in real time.
Note: By default, gadget specifications are cached for about an hour, so if you update your gadget XML file, you may not see the change on your Site page immediately. To bypass the cache while working on your gadget, add the parameter ?nocache=1 to the page URL in the address bar of your browser:
Here are my gadgets for Google Sites: