Improve validation for <link> tags.
Improve validation for <link> tags.

- Add a TODO to make rel= values mandatory for tags that intend to be
  specific, to avoid <link href="..."> from validating without a "rel=foo"
  attribute.
- Make type="text/css" optional for the author stylesheet tag.

https://github.com/ampproject/amphtml/issues/1205

erwinmombay/amphtml,dotandads/amphtml,nekodo/amphtml,Adtoma/amphtml,rajkumarsrk/amphtml,koshka/amphtml,mlb7687/amphtml,rlugojr/amphtml,jridgewell/amphtml,engtat/amphtml,fullspeed-sp/amphtml,luciancrasovan/amphtml,ericlindley-g/amphtml,adiant/amphtml,sriramkrish85/amphtml,shawnbuso/amphtml,DarXector/amphtml,dvoytenko/amphtml,jridgewell/amphtml,Mixpo/amphtml,Gregable/amphtml,sklobovskaya/amphtml,simplereach/amphtml,keithwrightbos/amphtml,smartadserver/amphtml,Re-lM/amphtml,rifler/amphtml,aghassemi/amphtml,spotxchange/amphtml,camelburrito/amphtml,zhucl/amphtml,NataliaLKB/amphtml,donttrustthisbot/amphtml,wjfang/amphtml,adtechnology/amphtml,camelburrito/amphtml,widespace-os/amphtml,kamtschatka/amphtml,burtcorp/amphtml,pietergreyling/amphtml,cvializ/amphtml,jdelhommeau/amphtml,vmarteev/amphtml,triplelift/amphtml,lzanol/amphtml,vmarteev/amphtml,aghassemi/amphtml,rsimha-amp/amphtml,engtat/amphtml,powdercloud/amphtml,Lith/amphtml,sharethrough/amphtml,DarXector/amphtml,zhangsu/amphtml,dotandads/amphtml,devjsun/amphtml,jridgewell/amphtml,koshka/amphtml,dvoytenko/amphtml,zhangsu/amphtml,triplelift/amphtml,chaveznvg/amphtml,danielrozenberg/amphtml,yevheniiminin/amphtml,src-code/amphtml,camelburrito/amphtml,donttrustthisbot/amphtml,rajkumarsrk/amphtml,avimehta/amphtml,smartadserver/amphtml,tdrl/amphtml,alanorozco/amphtml,chaveznvg/amphtml,smartadserver/amphtml,rsimha-amp/amphtml,BenDz/amphtml-atinternet,laterpay/amphtml,britice/amphtml,jridgewell/amphtml,jpettitt/amphtml,yandex-pcode/amphtml,ColombiaAnalytics/amphtml,kamtschatka/amphtml,Meggin/amphtml,britice/amphtml,techhtml/amphtml,arealit/amphtml,ADITION/amphtml,yandex-pcode/amphtml,mhc-gh/amphtml,taboola/amphtml,zhangsu/amphtml,yevheniiminin/amphtml,powdercloud/amphtml,sebastianbenz/amphtml,media-net/amphtml,nexxtv/amphtml,kmh287/amphtml,taboola/amphtml,pedjoni8/amphtml,amoad-amoe/amphtml,bpaduch/amphtml,kamtschatka/amphtml,jridgewell/amphtml,plista/amphtml,sebastianbenz/amphtml,pietergreyling/amphtml,dotandads/amphtml,wjfang/amphtml,zhucl/amphtml,ecoron/amphtml,JuliusThms/amphtml,zhouyx/amphtml,ampproject/amphtml,Lith/amphtml,jdelhommeau/amphtml,yandex-pcode/amphtml,zhucl/amphtml,honeybadgerdontcare/amphtml,lannka/amphtml,lzanol/amphtml,nathangoulding/amphtml,clawr/amphtml,gumgum/amphtml,arealit/amphtml,spotxchange/amphtml,techhtml/amphtml,BrightcoveOS/amphtml,BenDz/amphtml,engtat/amphtml,techhtml/amphtml,Re-lM/amphtml,sklobovskaya/amphtml,arealit/amphtml,Lith/amphtml,dotandads/amphtml,sklobovskaya/amphtml,NataliaLKB/amphtml,sharethrough/amphtml,arealit/amphtml,tlong2/amphtml,triplelift/amphtml,dvoytenko/amphtml,DistroScale/amphtml,comScore/amphtml,zhouyx/amphtml,comScore/amphtml,ericlindley-g/amphtml,pedjoni8/amphtml,engtat/amphtml,interspacejp/amphtml,ithinkihaveacat/amphtml,gumgum/amphtml,honeybadgerdontcare/amphtml,erwinmombay/amphtml,BrightcoveOS/amphtml,media-net/amphtml,powdercloud/amphtml,ruturajv-media-net/amphtml,interspacejp/amphtml,burtcorp/amphtml,alanorozco/amphtml,fullspeed-sp/amphtml,mlb7687/amphtml,NataliaLKB/amphtml,jasti/amphtml,yevheniiminin/amphtml,mkhatib/amphtml,yevheniiminin/amphtml,src-code/amphtml,BenDz/amphtml,amoad-amoe/amphtml,lzanol/amphtml,danielrozenberg/amphtml,rifler/amphtml,plista/amphtml,vmarteev/amphtml,mhc-gh/amphtml,bpaduch/amphtml,nexxtv/amphtml,jlucero-contentad/amphtml,dvoytenko/amphtml,sklobovskaya/amphtml,ampproject/amphtml,fullspeed-sp/amphtml,plista/amphtml,lzanol/amphtml,bpaduch/amphtml,smartadserver/amphtml,DarXector/amphtml,amoad-amoe/amphtml,jmadler/amphtml,ruturajv-media-net/amphtml,prateekbh/amphtml,yevheniiminin/amphtml,clawr/amphtml,spotxchange/amphtml,adup-tech/amphtml,fullspeed-sp/amphtml,taboola/amphtml,zhangsu/amphtml,yandex-pcode/amphtml,galeksic/amphtml,voyagegroup/amphtml,nathangoulding/amphtml,tdrl/amphtml,kmh287/amphtml,adtechnology/amphtml,cvializ/amphtml,pedjoni8/amphtml,NataliaLKB/amphtml,powdercloud/amphtml,powdercloud/amphtml,alanorozco/amphtml,arealit/amphtml,sebastianbenz/amphtml,shawnbuso/amphtml,adiant/amphtml,prateekbh/amphtml,Mixpo/amphtml,jasti/amphtml,kixer-com/amphtml,rajkumarsrk/amphtml,AaronCritchley/amphtml,cvializ/amphtml,cory-work/amphtml,Adtoma/amphtml,DistroScale/amphtml,comScore/amphtml,tdrl/amphtml,ecoron/amphtml,rifler/amphtml,src-code/amphtml,Re-lM/amphtml,zhouyx/amphtml,lannka/amphtml,DAC-f1h-dev/amphtml,yandex-pcode/amphtml,rifler/amphtml,tlong2/amphtml,yieldbot/amphtml,Mixpo/amphtml,vmarteev/amphtml,DAC-f1h-dev/amphtml,BenDz/amphtml-atinternet,Mobusi-ads/amphtml,wjfang/amphtml,tdrl/amphtml,ADITION/amphtml,Mobusi-ads/amphtml,nexxtv/amphtml,ecoron/amphtml,media-net/amphtml,devjsun/amphtml,jridgewell/amphtml,keithwrightbos/amphtml,ampproject/amphtml,kmh287/amphtml,ADITION/amphtml,yieldmo/amphtml,rlugojr/amphtml,DarXector/amphtml,BrightcoveOS/amphtml,Mixpo/amphtml,vmarteev/amphtml,luciancrasovan/amphtml,erwinmombay/amphtml,ericlindley-g/amphtml,Gregable/amphtml,erwinmombay/amphtml,criteo-forks/amphtml,clawr/amphtml,Mobusi-ads/amphtml,sortable/amphtml,prateekbh/amphtml,rlugojr/amphtml,pietergreyling/amphtml,yieldbot/amphtml,laterpay/amphtml,luciancrasovan/amphtml,Lith/amphtml,rajkumarsrk/amphtml,Mixpo/amphtml,bpaduch/amphtml,interspacejp/amphtml,DarXector/amphtml,mkhatib/amphtml,Adtoma/amphtml,nekodo/amphtml,voyagegroup/amphtml,dotandads/amphtml,Mobusi-ads/amphtml,camelburrito/amphtml,widespace-os/amphtml,voyagegroup/amphtml,tdrl/amphtml,media-net/amphtml,gumgum/amphtml,powdercloud/amphtml,camelburrito/amphtml,mkhatib/amphtml,sklobovskaya/amphtml,koshka/amphtml,src-code/amphtml,kmh287/amphtml,ithinkihaveacat/amphtml,voyagegroup/amphtml,criteo-forks/amphtml,zhucl/amphtml,ruturajv-media-net/amphtml,Gregable/amphtml,sortable/amphtml,mhc-gh/amphtml,dvoytenko/amphtml,jasti/amphtml,Lith/amphtml,BenDz/amphtml,jdelhommeau/amphtml,zhucl/amphtml,sriramkrish85/amphtml,kixer-com/amphtml,widespace-os/amphtml,engtat/amphtml,AaronCritchley/amphtml,shawnbuso/amphtml,Gregable/amphtml,jmadler/amphtml,Lith/amphtml,rajkumarsrk/amphtml,avimehta/amphtml,jpettitt/amphtml,zhouyx/amphtml,aghassemi/amphtml,amoad-amoe/amphtml,criteo-forks/amphtml,erwinmombay/amphtml,sortable/amphtml,honeybadgerdontcare/amphtml,triplelift/amphtml,ampproject/amphtml,rlugojr/amphtml,AaronCritchley/amphtml,Meggin/amphtml,jpettitt/amphtml,zhangsu/amphtml,keithwrightbos/amphtml,media-net/amphtml,loic-seguin/amphtml,DarXector/amphtml,laterpay/amphtml,rsimha-amp/amphtml,chaveznvg/amphtml,engtat/amphtml,pietergreyling/amphtml,nexxtv/amphtml,cory-work/amphtml,adtechnology/amphtml,interspacejp/amphtml,fullspeed-sp/amphtml,devjsun/amphtml,avimehta/amphtml,gumgum/amphtml,techhtml/amphtml,Meggin/amphtml,donttrustthisbot/amphtml,vmarteev/amphtml,wjfang/amphtml,wjfang/amphtml,Gregable/amphtml,danielrozenberg/amphtml,media-net/amphtml,jdelhommeau/amphtml,lannka/amphtml,rsimha-amp/amphtml,BrightcoveOS/amphtml,britice/amphtml,adiant/amphtml,src-code/amphtml,rsimha-amp/amphtml,lannka/amphtml,sebastianbenz/amphtml,adup-tech/amphtml,Mixpo/amphtml,burtcorp/amphtml,ithinkihaveacat/amphtml,yieldbot/amphtml,zhouyx/amphtml,JuliusThms/amphtml,vmarteev/amphtml,yieldmo/amphtml,galeksic/amphtml,laterpay/amphtml,jdelhommeau/amphtml,triplelift/amphtml,danielrozenberg/amphtml,sortable/amphtml,yieldmo/amphtml,mhc-gh/amphtml,nathangoulding/amphtml,yevheniiminin/amphtml,aghassemi/amphtml,jpettitt/amphtml,wjfang/amphtml,rifler/amphtml,mlb7687/amphtml,spotxchange/amphtml,aghassemi/amphtml,aghassemi/amphtml,comScore/amphtml,interspacejp/amphtml,comScore/amphtml,sriramkrish85/amphtml,chaveznvg/amphtml,zhangsu/amphtml,adup-tech/amphtml,plista/amphtml,JuliusThms/amphtml,pedjoni8/amphtml,bpaduch/amphtml,media-net/amphtml,luciancrasovan/amphtml,clawr/amphtml,sharethrough/amphtml,yandex-pcode/amphtml,gumgum/amphtml,ericlindley-g/amphtml,adtechnology/amphtml,clawr/amphtml,clawr/amphtml,mhc-gh/amphtml,Adtoma/amphtml,jpettitt/amphtml,gumgum/amphtml,clawr/amphtml,yieldmo/amphtml,avimehta/amphtml,BenDz/amphtml,luciancrasovan/amphtml,nexxtv/amphtml,adiant/amphtml,smartadserver/amphtml,loic-seguin/amphtml,JuliusThms/amphtml,jasti/amphtml,yieldmo/amphtml,jpettitt/amphtml,loic-seguin/amphtml,taboola/amphtml,criteo-forks/amphtml,cory-work/amphtml,keithwrightbos/amphtml,adiant/amphtml,kixer-com/amphtml,danielrozenberg/amphtml,jpettitt/amphtml,sharethrough/amphtml,triplelift/amphtml,adtechnology/amphtml,yieldbot/amphtml,rajkumarsrk/amphtml,honeybadgerdontcare/amphtml,keithwrightbos/amphtml,shawnbuso/amphtml,voyagegroup/amphtml,taboola/amphtml,galeksic/amphtml,adiant/amphtml,alanorozco/amphtml,avimehta/amphtml,donttrustthisbot/amphtml,chaveznvg/amphtml,amoad-amoe/amphtml,smartadserver/amphtml,taboola/amphtml,yieldmo/amphtml,criteo-forks/amphtml,mlb7687/amphtml,DAC-f1h-dev/amphtml,nexxtv/amphtml,BenDz/amphtml-atinternet,taboola/amphtml,DAC-f1h-dev/amphtml,wjfang/amphtml,rifler/amphtml,nexxtv/amphtml,adup-tech/amphtml,rlugojr/amphtml,shawnbuso/amphtml,jlucero-contentad/amphtml,Mobusi-ads/amphtml,mhc-gh/amphtml,ecoron/amphtml,Adtoma/amphtml,koshka/amphtml,prateekbh/amphtml,cvializ/amphtml,mkhatib/amphtml,tlong2/amphtml,BrightcoveOS/amphtml,ampproject/amphtml,engtat/amphtml,jmadler/amphtml,jlucero-contentad/amphtml,BenDz/amphtml,yieldmo/amphtml,devjsun/amphtml,rajkumarsrk/amphtml,tlong2/amphtml,cory-work/amphtml,Gregable/amphtml,yandex-pcode/amphtml,Mobusi-ads/amphtml,prateekbh/amphtml,pietergreyling/amphtml,kmh287/amphtml,tlong2/amphtml,dvoytenko/amphtml,lzanol/amphtml,BenDz/amphtml,keithwrightbos/amphtml,lannka/amphtml,ColombiaAnalytics/amphtml,kamtschatka/amphtml,AaronCritchley/amphtml,sriramkrish85/amphtml,danielrozenberg/amphtml,adup-tech/amphtml,donttrustthisbot/amphtml,jasti/amphtml,kixer-com/amphtml,interspacejp/amphtml,zhouyx/amphtml,techhtml/amphtml,sebastianbenz/amphtml,kixer-com/amphtml,prateekbh/amphtml,ADITION/amphtml,sharethrough/amphtml,Adtoma/amphtml,voyagegroup/amphtml,DistroScale/amphtml,alanorozco/amphtml,koshka/amphtml,jmadler/amphtml,rsimha-amp/amphtml,dotandads/amphtml,jpettitt/amphtml,britice/amphtml,Re-lM/amphtml,sklobovskaya/amphtml,burtcorp/amphtml,koshka/amphtml,ampproject/amphtml,chaveznvg/amphtml,ADITION/amphtml,Gregable/amphtml,spotxchange/amphtml,AaronCritchley/amphtml,media-net/amphtml,rsimha-amp/amphtml,jasti/amphtml,loic-seguin/amphtml,donttrustthisbot/amphtml,mhc-gh/amphtml,alanorozco/amphtml,burtcorp/amphtml,honeybadgerdontcare/amphtml,devjsun/amphtml,widespace-os/amphtml,DistroScale/amphtml,techhtml/amphtml,erwinmombay/amphtml,spotxchange/amphtml,zhouyx/amphtml,simplereach/amphtml,sriramkrish85/amphtml,shawnbuso/amphtml,jmadler/amphtml,nathangoulding/amphtml,gumgum/amphtml,tdrl/amphtml,sortable/amphtml,galeksic/amphtml,ampproject/amphtml,adiant/amphtml,mlb7687/amphtml,camelburrito/amphtml,donttrustthisbot/amphtml,adup-tech/amphtml,ruturajv-media-net/amphtml,lannka/amphtml,cvializ/amphtml,burtcorp/amphtml,arealit/amphtml,yieldbot/amphtml,kmh287/amphtml,honeybadgerdontcare/amphtml,sebastianbenz/amphtml,pietergreyling/amphtml,widespace-os/amphtml,jridgewell/amphtml,nekodo/amphtml,voyagegroup/amphtml,mlb7687/amphtml,sortable/amphtml,jlucero-contentad/amphtml,bpaduch/amphtml,pedjoni8/amphtml,zhucl/amphtml,cory-work/amphtml,BrightcoveOS/amphtml,laterpay/amphtml,sharethrough/amphtml,pedjoni8/amphtml,galeksic/amphtml,jmadler/amphtml,simplereach/amphtml,cvializ/amphtml,alanorozco/amphtml,ruturajv-media-net/amphtml,ecoron/amphtml,ampproject/amphtml,ericlindley-g/amphtml,AaronCritchley/amphtml,bpaduch/amphtml,ecoron/amphtml,prateekbh/amphtml,keithwrightbos/amphtml,yieldbot/amphtml,src-code/amphtml,alanorozco/amphtml,adtechnology/amphtml,ericlindley-g/amphtml,danielrozenberg/amphtml,DAC-f1h-dev/amphtml,sklobovskaya/amphtml,luciancrasovan/amphtml,JuliusThms/amphtml,mkhatib/amphtml,cory-work/amphtml,honeybadgerdontcare/amphtml,jlucero-contentad/amphtml,devjsun/amphtml,amoad-amoe/amphtml,shawnbuso/amphtml,kixer-com/amphtml,avimehta/amphtml,kamtschatka/amphtml,danielrozenberg/amphtml,criteo-forks/amphtml,kamtschatka/amphtml,cory-work/amphtml,zhucl/amphtml,luciancrasovan/amphtml,kixer-com/amphtml,nekodo/amphtml,fullspeed-sp/amphtml,DAC-f1h-dev/amphtml,ithinkihaveacat/amphtml,camelburrito/amphtml,ColombiaAnalytics/amphtml,tlong2/amphtml,comScore/amphtml,arealit/amphtml,jdelhommeau/amphtml,DistroScale/amphtml,laterpay/amphtml,ColombiaAnalytics/amphtml,comScore/amphtml,rlugojr/amphtml,DistroScale/amphtml,lannka/amphtml,jlucero-contentad/amphtml,powdercloud/amphtml,widespace-os/amphtml,sriramkrish85/amphtml,ruturajv-media-net/amphtml,mkhatib/amphtml,mhc-gh/amphtml,lzanol/amphtml,Gregable/amphtml,JuliusThms/amphtml