I recently converted all my .html files to .php, and while the stylesheet they're linked to (which hasn't changed) works almost perfectly, the <h1> through <h6> tags (headings) don't work with the new .php files. The font appears the same size as the body, which is too small to be legible with the font I chose for my article titles and subtitles, which use <h1> and <h2>. It's strange; when I update any class at all, the changes appear in-browser on refresh, as they should. But not so for the headings.
Any advice?

can you post an example? link or sample code, something we can test.

Actually, I've just solved the issue. My style.css file — a result of a merger of all my other CSS files which, incidentally, I did when converting my site to PHP — contained contradicting classes.

<?php header ('content-type:text/css');
ob_start("ob_gzhandler"); ?>
/* <style type=text/css> */ 
@media all {.dontall {display:none;}}
@media handheld { img { display:none; } }
@media print {
 body {font-size:10pt; font-family:verdana, arial, sans-serif;}
 .dontprint {display:none;}
 .lt {float:left;text-align:right;width:30%;}
 .rt {float:right;text-align:left;width:65%;}
 .bk {clear:both;width:97%;border:#778899 solid 0;margin:4pt 1% 4pt 1%;padding:4pt;}
 .bkb {width:95%;border:#778899 solid 1pt;margin:4pt 1% 4pt 1%;padding:4pt;}
 .block {float:left;border:#778899 solid 1pt;margin:5pt;padding:2pt;}
 .blockr {float:right;text-align:left;border:#778899 solid 1px;margin:5pxpadding:5px;}
 fieldset {border:none;}
 p {padding:1px;margin:0;list-style-type:square;}
 p:first-line {font-weight:bold;}
 p:first-letter {font-size:100%;}
@media screen {
 .dontshow{ display:none;} 
 .doshow{ display:inline;}
a {text-decoration:none;padding:0 3px;}
a:hover {background-color:#66cdaa;text-decoration:none;}
a:focus {background-color:#66cdaa;text-decoration:none;}
<?php ob_flush();?>

the first 2 and last 1 lines, style.css as style.css.php