Wikitext is similar to Markdown and dissimilar to C in that the grammar is loose. Any invalid token combinations are rendered as-is, rather than producing a fatal parsing error which halts the process. This latter, C-like (or any programming language, really) philosophy was how the original version of ftml operated. However this presents obvious incompatibilities with Wikidot, and the grammar had to be increasingly complicated to handle edge-case conditions.
ftml as it exists now performs preprocessing substitutions and tokenization like the first version, but has a hand-written parser which explicitly permits loose fallback rules. Thus, any invalid token formations are interpreted as the raw text itself, rather than being forced to fail parsing completely.
More specifically, for each encountered token, the parser will attempt to match the first rule which expects it. Incoming tokens will be handled, producing elements, or until an invalid token is received, at which point a "warning" will be produced and this rule will abort.
Following this, the parser will attempt to apply the second rule (if any), etc., until all rules are exhausted. At this point, if no match can be made, the default "fallback" rule is applied. This is the case where all the tokens are interpreted literally.
For any tokens which are successfully consumed to produce elements, the pointer to the remaining tokens is bumped up (really, a later subslice is taken), and the element is appended to the final list. Note that this operation is applied recursively, so that any containers (elements which contain other elements) will perform this same operation to populate themselves.
In accordance to the Wikidot parsing strategy, all "warnings" are non-fatal. (These were previously named "errors" but were renamed in #103 for clarity). In the worst-case scenario, all tokens fail all rules, and all are parsed with the fallback, rule, producing a warning for each incident. In a more typical case, any invalid structures will produce warning, and will parsed as best it can.
These warnings are returned to the caller to provide information on where the process failed, while still producing the fallback render. This provides the both of best worlds: warnings to assist with wikitext debugging, but also not hard-failing rendering in case of one.