Otóż postanowiłem sprawdzić, czy moja próbna strona jest zgodna z xhtml, i oto wyniki:
Warning Line 26 column 55: cannot generate system identifier for general entity "category".

[HTML] pobierz, plaintext 
<li><a href='index.php?id=articles&category=cs'>Counter Strike</a></li>
[HTML] pobierz, plaintext 

An entity reference was found in the document, but there is no reference by that name defined. Often this is caused by misspelling the reference name, unencoded ampersands, or by leaving off the trailing semicolon (. The most common cause of this error is unencoded ampersands in URLs as described by the WDG in "Ampersands in URLs".

Entity references start with an ampersand (&) and end with a semicolon (. If you want to use a literal ampersand in your document you must encode it as "&" (even inside URLs!). Be careful to end entity references with a semicolon or your entity reference may get interpreted in connection with the following text. Also keep in mind that named entity references are case-sensitive; &Aelig; and ć are different characters.

If this error appears in some markup generated by PHP's session handling code, this article has explanations and solutions to your problem.

Note that in most documents, errors related to entity references will trigger up to 5 separate messages from the Validator. Usually these will all disappear when the original problem is fixed.

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# Error Line 26 column 55: general entity "category" not defined and no default entity.

[HTML] pobierz, plaintext 
<li><a href='grajta/index.php?id=articles&category=cs'>Counter Strike</a></li>
[HTML] pobierz, plaintext 

This is usually a cascading error caused by a an undefined entity reference or use of an unencoded ampersand (&) in an URL or body text. See the previous message for further details.
" title="Zobacz w manualu PHP" target="_manual"># Warning Line 26 column 55: cannot generate system identifier for general entity "category".

[HTML] pobierz, plaintext 
<li><a href='index.php?id=articles&category=cs'>Counter Strike</a></li>
[HTML] pobierz, plaintext 

An entity reference was found in the document, but there is no reference by that name defined. Often this is caused by misspelling the reference name, unencoded ampersands, or by leaving off the trailing semicolon (. The most common cause of this error is unencoded ampersands in URLs as described by the WDG in "Ampersands in URLs".

Entity references start with an ampersand (&) and end with a semicolon (. If you want to use a literal ampersand in your document you must encode it as "&" (even inside URLs!). Be careful to end entity references with a semicolon or your entity reference may get interpreted in connection with the following text. Also keep in mind that named entity references are case-sensitive; &Aelig; and ć are different characters.

If this error appears in some markup generated by PHP's session handling code, this article has explanations and solutions to your problem.

Note that in most documents, errors related to entity references will trigger up to 5 separate messages from the Validator. Usually these will all disappear when the original problem is fixed.

✉
# Error Line 26 column 55: general entity "category" not defined and no default entity.

[HTML] pobierz, plaintext 
<li><a href='grajta/index.php?id=articles&category=cs'>Counter Strike</a></li>
[HTML] pobierz, plaintext 

This is usually a cascading error caused by a an undefined entity reference or use of an unencoded ampersand (&) in an URL or body text. See the previous message for further details.

Kompletnie nie wiem co to oznacza, czy ktoś mógłby mi przybliżyć co robie źle?

'&' (unencoded ampersand (&)) musisz zapisywac jako '&' (Entity references start with an ampersand (&) and end with a semicolon)

Dzięki, już jest ok ;]