to get the help screen for tidy type: [ tidy -help ]
If you have an html file you want to convert to xhtml do this:
[tidy -asxhtml -m filename.html]

---

On my system this is the result of the above command:

tidy [option...] [file...] [option...] [file...]
Utility to clean up and pretty print HTML/XHTML/XML
see http://tidy.sourceforge.net/

Options for HTML Tidy for Linux/x86 released on 1st September 2004:

File manipulation
-----------------
  -out or -o <file> specify the output markup file
  -config <file>    set configuration options from the specified <file>
  -f      <file>    write errors to the specified <file>
  -modify or -m     modify the original input files

Processing directives
---------------------
  -indent  or -i    indent element content
  -wrap <column>    wrap text at the specified <column> (default is 68)
  -upper   or -u    force tags to upper case (default is lower case)
  -clean   or -c    replace FONT, NOBR and CENTER tags by CSS
  -bare    or -b    strip out smart quotes and em dashes, etc.
  -numeric or -n    output numeric rather than named entities
  -errors  or -e    only show errors
  -quiet   or -q    suppress nonessential output
  -omit             omit optional end tags
  -xml              specify the input is well formed XML
  -asxml            convert HTML to well formed XHTML
  -asxhtml          convert HTML to well formed XHTML
  -ashtml           force XHTML to well formed HTML
  -access <level>   do additional accessibility checks (<level> = 1, 2, 3)

Character encodings
-------------------
  -raw              output values above 127 without conversion to entities
  -ascii            use US-ASCII for output, ISO-8859-1 for input
  -latin0           use US-ASCII for output, ISO-8859-15 for input
  -latin1           use ISO-8859-1 for both input and output
  -iso2022          use ISO-2022 for both input and output
  -utf8             use UTF-8 for both input and output
  -mac              use MacRoman for input, US-ASCII for output
  -win1252          use Windows-1252 for input, US-ASCII for output
  -ibm858           use IBM-858 (CP850+Euro) for input, US-ASCII for output
  -utf16le          use UTF-16LE for both input and output
  -utf16be          use UTF-16BE for both input and output
  -utf16            use UTF-16 for both input and output
  -big5             use Big5 for both input and output
  -shiftjis         use Shift_JIS for both input and output
  -language <lang>  set the two-letter language code <lang> (for future use)

Miscellaneous
-------------
  -version  or -v   show the version of Tidy
  -help, -h or -?   list the command line options
  -help-config      list all configuration options
  -show-config      list the current configuration settings

Use --blah blarg for any configuration option "blah" with argument "blarg"

Input/Output default to stdin/stdout respectively
Single letter options apart from -f may be combined
as in:  tidy -f errs.txt -imu foo.html
For further info on HTML see http://www.w3.org/MarkUp