Such files as HTML have no characteristic signatures but may be identified by indirect indicators since they contain ASCII symbols.
File size determination
To recover a file it is not enough to find where it begins. To complete data recovery OS must “know” where this file ends. File’s end may be found only if its size is known. File size is determined by analysis of the file header (ZIP, JPEG, AVI, etc.) or by analysis of sectors of the disk platter which immediately follow the file beginning. For example, the signature-based search algorithm will identify the sector as the HTML or TXT file’s end if this sector doesn’t contain symbols from ASCII table.
Signature-based search is not cure-all solution. Re-recorded and fragmented files (especially large files) are probably one of the biggest headaches in data recovery.
Modern data recovery tools employ hybrid approach to the analysis of the disk sectors. The main idea of which is to retrieve as much information from the file system as possible and to use signature-based search as little as possible – only by damaged or failed file system or by search of data deleted long time ago.