Takoboto is a Japanese dictionary website, but they also provide grammar references. The grammar search portion of Takoboto allows users to look for specific grammar points. Once a particular grammar pattern has been found and selected, it provides a general meaning, formation (what kind of word it is connected to and how it's used), and example sentences that demonstrate the grammar pattern in use. It also shows the JLPT level that corresponds to each point to give you an idea of how basic/advanced that the grammar point is.
Most of their content is based-on Creative Common license content, content is made available for sharing to the public. For example, the grammar content comes from ジェイグラム, a Japanese learning community / user-generated grammar database, which is no longer available. And many example sentences come from the sentence database Tatoeba and the Tanaka Corpus.
Takoboto is truly just a database of grammar. That being said, it does not seem to have the potential to be treated as an online textbook. But if you already have a grammar pattern in mind that you would like to know more about, then opening up Takoboto is the way to go.