Yes, and No!
It does keep the main pages as html, however FrontPage keeps a certain structure and follow-up information that has to be supported by a special arrangement of additional directories, otherwise it get corrupted. The extensions provide these directories on Apache servers.
If you are using FrontPage and you are not using certain functions, it is possible to use it without extensions. It is not full proof but some people risk it.
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To totally full proof the usage of FrontPage it has to be published on Win200 Server.
Hosting companies that offer Win2000 servers are usually more expensive.
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The following is a list of FrontPage functions that need the FrontPage Server Extensions.
FrontPage created server-side Image Maps
WebBot Confirmation Field component
WebBot Discussion component
WebBot Registration components
WebBot Save Results component
WebBot Scheduled Image
WebBot Scheduled Include component
WebBot Search component
WebBot HTML Markup component
WebBot Include component
WebBot Substitution component
Database Results Wizard
Send To Database Form Handler
Nested Sub webs
Lightweight Source Control (Document Check-in/Check-out without VSS)
Categories Component
StyleSheet links to multiple files or ASP files
Confirmation Field
Discussion Form Handler
FrontPage created server-side Image Maps
Hit Counter
Registration Form Handler
Save Results Form Handler
Search Form
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More detailed info here:
How to Use FrontPage on Entry Level Hosting service?