What is HTML?
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:
Tutorials and Courses – W3C
Learn HTML | MDN
HTML tutorial – The Basics
What are gadgets?
Click Edit page ► Insert ► More gadgets to view and search thousands of existing gadgets in the