Hi Shanil
Who told that most of the times HF oscillators are based on series resonant mode ?
I think you'd better to refer to this fantastic book for crystal oscillator design :
Crystal oscillator circuit written by Robert J Matthys . it will answer all of your questions about crystal and crystal oscillators as well .
Anywhere , you can see my blog , for more informations too . take a look below , please :
https://www.edaboard.com/blog/1788/
About the specifications of series mode of resonant for a crystal , we can mention :
1- phase shift = zero degree
2- it will pass our desired frequency .
Hence we can easily use it in feedback path . ( for oscillators in positive feedback path )
Best Wishes
Goldsmith