How do I add typekit fonts to a D8 theme?
For put the library to the header you must user header: true, then:

mytheme.info.yml

libraries:
  - mytheme/typekit


mytheme.libraries.yml:

typekit:
  version: 1.x
  header: true
  js:
    //use.typekit.net/abc1abc.js: { every_page: true }
    js/custom.js: { every_page: true }


js/custom.js:

try { 
  Typekit.load(); 
} 
catch(e) { 
}