I guess, in terms of preservation, about the same rules apply as for a citation and/or block quote. Meaning, clearly state that this is a quote and provide references to the original (so it can be identified). Include a statement of good faith, mentioning your willingness to revoke the content, should conflicts arise. (Also state why you found it important to preserve this, so it can be understood as a tribute to the original.) Do not monetize in any way.
The difficult question here is, if it's really a matter of preservation. The site/page isn't actually gone, it's just the certificate that has expired. The original content is still reachable, while jumping through a few hoops (i.e. browser dialogs). Still, following the guidelines provided above should somewhat mitigate the impact of any wrong doing.
The difficult question here is, if it's really a matter of preservation. The site/page isn't actually gone, it's just the certificate that has expired. The original content is still reachable, while jumping through a few hoops (i.e. browser dialogs). Still, following the guidelines provided above should somewhat mitigate the impact of any wrong doing.