Can web developers use HTML 5 to obtain your search histories (through temporary files, cookies. etc.) retroactively?

HTML 5 has nothing to do with it. The original purpose of a cookie was to save settings etc so the next time you visited the site it could pull those files and the ability for someone to grab that file to exploit what you have been up to has been around since the inception of the internet as we know it

@mrlaconic So, does this mean it’s inevitable that third party companies possess, and are going to sell, our lifetime internet search histories to future employers, the government, etc. as public record? Ahh!

The amount of traffic flying around on the internet is so enormous it seems unlikely a data miner would really benefit from looking at all the sites you’ve visited. It’s really hard to intercept traffic between two computers, especially when you have soooo many to do it to. There are security protocols in place to prevent such interceptions. I also don’t believe it’s possible to get your browsing history retroactively as you asked, especially if you delete your browsing history on a regular basis. Unless you give them specific access to your computer, websites can’t really access anything outside the confines of your browser anyway. Cookies are stored by the browser and certainly don’t store all your browsing history – just bits of information the website needs to give you a consistent experience.