Working with documents in electronic format is inherently different fr
om dealing with materials in print; nor can all electronic formats be
considered equivalent. Processing and presenting SGML is not the same
as processing and presenting materials in other markup or word process
ing formats. To maximize flexibility and extensibility, SGML is highly
modular, which complicates implementation. Its emphasis on content st
ructure rather than appearance enhances searchability but makes consis
tent and precise display difficult. Mechanisms used to maximize platfo
rm and software independence (e.g., entities, link protocols), though
effective, can be used incorrectly or in ways difficult to implement o
n some systems or using certain software. Difficult questions remain f
or libraries planning to implement SGML.