Distractions. I am text-oriented and easily distracted, so any auto-playing animation, video, audio, pop-out, or basically anything that moves on the display and causes distraction annoys me.

Static images and graphics are fine, and user-triggered movement is fine, but the all-dancing, all-singing, in-your-face sites can please take a flying leap at a rolling doughnut.

TBH, I even find distracting text annoying - Stack Overflow's "Hot Network Questions" is a good example. I'm looking at the details of how to parse version N of format X, and it is not at all helpful when my eye catches: "If a dwarf barbarian wears heavy armor how fast do they move?" (actual HNQ question, no offense intended to the querent, dwarves, barbarians, etc.)

Prompts for newsletter, login (ala glass door), notifications, etc

Banking sites that still use verification questions

Landing pages that don't take you to the actual content (such as if you type drive.google.com into the url bar when you're logged in to Google and have been going to the same site for a while)

Lots of Tracking

- Blank, white page when JavaScript is disabled

- Basic functionality broken when JavaScript is disabled

- Unnecessary use of iFrames and other subdocument elements

- Loading megabytes of libraries and scripts for a basic menu animation that could be done with CSS3 or plain JS

- Taking me off-domain to log in - the logged in area should be dashboard.example.tld, not exampledashboard.tld (mainly banking sites do this)

- Requiring logging in after visiting an email address verification link

- Sending email verification links, I would really prefer verification codes that don't require visiting a link

- HTTPS mixed content errors

- Messy HTTP response headers, such as invalid values, duplicate headers, or default values for everything

- No easy way to find a contact method for the security team, such as the RFC2142 security@ email address, or security.txt

- Nags to install apps on mobile (ex: opening reddit on mobile)

- Videos autoplaying on some news sites

- Cookie prompts

- And the worst: overlays that appear on scrolling

Forcing the mobile version when browsing on a cell phone. I may be in the minority here, but I prefer having the full site available. If I wanted the mobile version by default I would just use an app.

Everything that is not the content I came for

Splash dialogs asking me for my email address, or to sign up on a site where I've just come to read one article.

Having to dismiss something is an utter pain.

Any kind of float trying to prevent the content to be fully displayed on the screen. Any distraction, ad, even recommendations.

Sticky headers. Even worse on small phone screens.

Asking me to install the app on every page after I have said no. I deliberately avoid reddit links on my phone now.

Those chatbot/assistant pop-ups. 10x worse when they make that 'pop' sound.

Asking for browser notifications.

Missing functions in the mobile site.

Too much white space. (Ie new reddit)

PWAs. If I wanted an app, I would be on the store and not in the browser.

Anything that is not the content.

In order of annoyance:

Popover prompts (this catches a lot), autoplay, scroll-jacking, ads, cookie notification, geo-blocked, paywalled, pop-ups, middle-click for open new tap does something else or does not work because it's no real link

- When companies put fake graphics on their websites. E.g. the typical small startup who puts corporate/big business graphics in their main web site, rather than real pictures of themselves/their office. Pathetic.

- Asking for registration/login, just to get spammed later with annoying newsletter bullshit.

- Everything else others said already.

1. Quora paywall

2. Reddit install app nag

3. Paywalls on media/news sites