Idempotence ([...]) is the property of certain operations in mathematics and computer science whereby they can be applied multiple times without changing the result beyond the initial application.
數學定義是這樣跑：
An element x of a magma (M, •) is said to be idempotent if:
x • x = x.
If all elements are idempotent with respect to •, then • is called idempotent. The formula ∀x, x • x = x is called the idempotency law for •.
A request method is considered "idempotent" if the intended effect on the server of multiple identical requests with that method is the same as the effect for a single such request. Of the request methods defined by this specification, PUT, DELETE, and safe request methods are idempotent.
Request methods are considered "safe" if their defined semantics are essentially read-only; i.e., the client does not request, and does not expect, any state change on the origin server as a result of applying a safe method to a target resource. Likewise, reasonable use of a safe method is not expected to cause any harm, loss of property, or unusual burden on the origin server.
然後標準的 HTTP method 是有定義的：
+---------+------+------------+---------------+
| Method | Safe | Idempotent | Reference |
+---------+------+------------+---------------+
| CONNECT | no | no | Section 4.3.6 |
| DELETE | no | yes | Section 4.3.5 |
| GET | yes | yes | Section 4.3.1 |
| HEAD | yes | yes | Section 4.3.2 |
| OPTIONS | yes | yes | Section 4.3.7 |
| POST | no | no | Section 4.3.3 |
| PUT | no | yes | Section 4.3.4 |
| TRACE | yes | yes | Section 4.3.8 |
+---------+------+------------+---------------+
“Log out” links that should be forms with a “log out” button—you can always style it to look like a link if you want.
“Unsubscribe” links in emails that immediately trigger the action of unsubscribing instead of going to a form where the POST method does the unsubscribing. I realise that this turns unsubscribing into a two-step process, which is a bit annoying from a usability point of view, but a destructive action should never be baked into a GET request.
這兩個動作都會造成 server 端的狀態改變，不應該用 GET，而我自己常常忘記第一個... 這邊其實可以用 form 產生 POST 需求，並且用 css 效果包起來，達到看起來跟一般的連結一樣。
Googlebot uses a web rendering service (WRS) that is based on Chrome 41 (M41). Generally, WRS supports the same web platform features and capabilities that the Chrome version it uses — for a full list refer to chromestatus.com, or use the compare function on caniuse.com.
裡面提到一些值得注意的事情，像是不支援 WebSocket，所以對於考慮 Google 搜尋結果的頁面來說，就要注意錯誤處理了...
