The link didn't show me an image. It said error found, but I thought from the beginning that they were dandelion seedlings. They must change when mature, with the leaves changing shape. I have smothered them with fall leaves and wood chips on top of that in most of my mulch areas under trees and areas that will soon have perennials. In the growing season I hit them with a few drops of round-up, but they aren't actively growing and may not respond to round-up. I know lots of folks don't approve of useing weed killer but I have so much area to kill weeds in that I use round-up once a week to keep ahead of the weeds, which are 90 % dandelion leaf/rosettes. There is the occasional crabgrass or purslane, but these leaves seem to come along each spring and late summer/fall. I used to like them being there when I had bunnies as pets, they loved eating them. I picked them, lots but of course not all of them. I have gotten some corn gluten for use in spring to avoid the spring flush.

If I were a gamblin' woman, I would bet on the weed: Sonchus oleraceus, annual cow thistle.It has a strong tap root that makes digging out very difficult. And they will provide you with plants for years to come because they are heavy-duty self-seeders. You don't want them in your flowerbeds. They can grow to 3-4ft tall.

Agree that is Sonchus, which often turns color in the fall. I cropped your photo for a closer view, but could not see enough leaf detail to determine species. However, you can easily do that yourself. If the leaf base auricles are rounded, as shown in the first photo below, your plant is Sonchus asper. If the leaf base auricles are sagittate (2nd photo), your plant is Sonchus oleraceus.

Definitely Lactuca. My wife loves them and won't let me weed them out. She's an artist and loves the 10-12 foot high flowering stalks as a garden accent, and then I have a million seedlings the next year. LOL I would agree that they are 2 different species of plant versus original photo.