What scriptures are considered valid?
All Hindu scriptures (Shruti, Smriti etc.) are valid and authentic source. Works of Acharyas, Sayings of Saints, Swami and Gurus are also valid source to cite.
What sources are considered valid?
Now talking about citing texts from websites.
Websites that provide English translation and commentary of Hindu scriptures:
Hindu scriptures are primarily written in Sanskrit which are translated into various languages and most of websites provide English translation of it. For example Here you'll find many useful resources to get scriptures. Citing English translation and commentary of scriptures from websites is valid as they are based on (or affiliated with) Hindu scriptures.
Which website we should prefer to cite? For citing scriptures, we shoud prefer websites that provides proper reference to scripture i.e verse-to-verse translation e.g this so-that you can give proper reference e.g Rigveda 2.3.4, BG 5.23 etc. But if I say Manu Smriti says xyz or Krishna said xyz without proper reference like Manu. 3.22. then it is improper and missing reference which we can't rely on. So, we should not cite such texts from websites that are actually "missing proper reference".
There are many sites provide English translation and interpretation of scriptures besides we do know; We can't pre-judge the correctness of translation and/or interpretation of them. Voting on answers will decide the correctness of it.
Websites that provides modern works of Acharyas:
- We should prefer well-known and/or official websites for citing works/saying of Acharyas/Swami/Saints. For example Belur Math for Works of Swami Vivekananda and Sringeri and Kamakoti for Adi Shankaracharya. Similarly reputed websites for Ramanujacharya and other Acharyas/Swami/Saint.
Similarly for websites that provides various article on Hindu practices, we should prefer reputed website but as we can't pre-judge the correctness and reliability of all sites, we should not disallow websites for citing as reference since we don't have a list of approved websites by community (We can implement restriction if we provide and maintain list of approved sites). Depending on the content (expectation according to question), voting will serve the purpose of deciding correctness of information being cited and provided in answers.
In brief, listing with descending order of authenticity, 1. Hindu Scriptures 2.Works of Acharyas/Swami/Saints, 3. Information cited from reputed websites are considered valid sources.
Note that Blogs are not considered valid source as they are mostly nothing but the opinions of blogger and we can't rely on mere personal opinions or personal experience. Similarly your idea of super-sacred-texts.com with your personal/own philosophy will fall in this category.