Recent Update to HTML Box on classic Sites
If your HTML Box has recently stopped working here's what you need to do:
Go to a page where you have an HTML Box
Edit the page
Open the HTML box and you will see the new HTML Box
Set the height and width to something sensible to fit your content
Close the HTML Box
Save the page
Repeat for each page/HTML box that is not working.
Add an HTML Box to classic Google Sites
Use the Insert menu, then HTML Box.
Once the Insert HTML Box editor appears add the code you want to use.
Google Sites checks the code, while you edit, for errors and will not add the code to the page until correct.
Once finished use the Save button on the Insert HTML Box.
Use the Save button to save the page and view the page with your new code.
Site Search Box anywhere on the page
Insert a HTML Box as described above in steps 1 to 3
Copy and paste the code above to make a Site Search box (make sure you add the full site URL to the action attribute)
Use the Save button on the Insert HTML Box.
Use the Save button to save the page and view the code.
You will see a search box that looks just like the site search box, a working example follows (remember this is not a gadget and is on the page).
Working Classic Google Site Search Box
Not everything is possible but it brings a whole new world of possibilities.