You may see the basic HTML version of Gmail if:
- You’re using an unsupported browser.
- You’re using a slow network connection.
- You previously set basic HTML as your default view. Click Standard View at the top right of Gmail to switch to the latest version.
- You were trying to access the older version of Gmail, which we retired in October 2010.
Maintaining multiple versions of Gmail adds significant maintenance costs and makes it tougher for us to fix bugs, release new features, and make Gmail even faster. The older version of Gmail lacked core features like chat, voice and video chat, and label improvements, and the vast majority of Gmail users no longer used it. We’re keeping the basic HTML version around to allow people with slow network connections and unsupported browsers to access their accounts, but we recommend using the current version of Gmail for the best experience. [source]