What is Gmail clipping?
Gmail clipping is when Gmail cuts a long email short and shows [Message clipped] with a link to view the entire message. Google does not document the limit; senders widely observe it at about 102 KB of HTML. What falls past the cut, often the footer and its unsubscribe link, stays hidden until the reader clicks.
Reading the result
Paste the HTML of your email. The checker counts its size in bytes as UTF-8, compares it with 102,000 bytes and with the 80 KB that Email on Acid suggests as a margin, and lists what the bytes are spent on: inline styles, style blocks, embedded base64 images, comments, link addresses and extra whitespace. Nothing you paste leaves your browser.
Common problems it finds
Images embedded as base64
An image embedded as a data: URI puts the whole file into the HTML, a third larger once encoded as base64. Host the image and link to it instead.
The same inline styles on every cell
Email builders write every CSS rule onto every element so that each client renders it, and the same declarations repeat hundreds of times. Removing unused declarations and whitespace helps; the styles themselves are safest left inline.
Tracking added at send time
Your platform rewrites each link into a longer tracking address and adds code of its own, so the message sent is larger than the HTML you designed. Leave headroom, or paste the HTML of a test message you received.
Comments left by an editor
Builder notes and commented-out sections count toward the size. Outlook’s conditional comments (<!--[if mso]>) are another matter: Outlook reads them, so keep them.