Rebuilding landing pages inside HubSpot is pretty straight forward because they give you access to a blank html page with some dynamic fields (page title, header footer scripts styles, tracking, etc). So if have a page built on another platform you sometimes can reuse the HTML and CSS styles with some minor tweaking to get it to work.
"Can Hubspot support hosting custom coded HTML?" Could you please clarify your question. Do you mean customer support for additional tweaks and customizations?
I will be tagging some of our top experts on this matter to give you their best advice, @DavidFJones, @mmetros, @SandyG1, could you share some thoughts to @mmetros question?