Reference
What is ETag?
An ETag is an identifier a server sends with a response so a browser can ask later whether the resource has changed. If it has not, the server replies 304 Not Modified with no body, saving the entire download.
Also called: entity tag · conditional request
Where it helps
On HTML, which usually cannot be cached for long because it changes. An ETag lets a repeat visitor revalidate cheaply instead of re-downloading the whole document — a meaningful saving on a slow connection.
The configuration mistake
Serving HTML with no-cache and no ETag means every repeat visit pays a full download and a full round trip. It is a common shared-hosting default, and it shows up as a slow repeat-visit experience that no front-end work explains.
When to remove them
On fingerprinted assets with a year-long immutable cache, an ETag is redundant — the filename already guarantees the content. Some multi-server setups also generate inconsistent ETags per node, which defeats the purpose entirely.
Where this is covered in depth
A definition can only go so far. Core Web Vitals, and the order that actually reduces them covers this properly — 3 minutes, free, no email required.
Who wrote this
Anas Bin Masud builds e-commerce sites and does technical SEO for businesses in the UK, Canada and Pakistan. These definitions come from client work rather than from a content brief — where an entry describes a mistake, it is usually one found on a real site. More about how I work.