[1] Resource Hints (2016-05-21 01:13:13 +09:00) https://w3c.github.io/resource-hints/#h-hint-probability-pr-attribute

[2] Removed the `pr` attribute definition and examples (yoavweiss著, 2018-01-15 18:58:56 +09:00) https://github.com/w3c/resource-hints/commit/9769790bdc00d7d7f747cc26f60a0131f851d1db

[3] Removed the `pr` attribute definition and examples by yoavweiss · Pull Request #76 · w3c/resource-hints (2018-01-16 11:06:21 +09:00) https://github.com/w3c/resource-hints/pull/76