Joe Offer wrote: It appears to be safe to click the "close" button on the full-page ads...

Trouble is, that goes against what should be an instinctive reaction - to never click a button on an uninvited pop-up. If people do that here, they'll do it on other sites. Ads can contain malware - 'malvertising' - and a fake 'X' close button is a common trick. The only 'safe' thing to do is close the tab. The safest thing to is to block them.

I've only seen these on my iPhone, which has a 4" screen. They certainly make mudcat unusable on that, but I don't use it for web browsing. I don't know what iThings allow ad-blockers - older ones don't.

As Helen said, if you block ads some sites will object and require you to turn the blocker off. For those sites you choose whether to comply; ad blockers allow you to whitelist specific sites.