Even from browsers that Gmail supports, such as Chrome, Edge, and Firefox, you can display Gmail in simple HTML format by clicking 'Load in simple HTML format' when opening Gmail.

Gmail's basic HTML view will disappear in January 2024 Google hasn't made any public announcement of this change, instead quietly adding a disclaimer on its support page.

Google: Goodbye Gmail’s Basic HTML View—Gone by 2024 But Why? The basic Gmail for slow and outdated browsers will soon bid farewell.