They both have the MIT license, which may be a so-called “permissive” licence, but it does require attribution. Did they base asm-dom on Snabbdom? That’s perfectly fine. Did they remove the copyright notice? That’s illegal.
Is in the example the usual place to put this?
MIT specifically calls out that the notice must be included in copies of "substantial portions" of the software which makes no sense if the entirety of the notice is already part of every file.
> The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
If someone has more information or links about that, please let me know