I have Drupal 6 Site.
Here are my Urls:-
<link rel="stylesheet" type="text/css" href="<?php echo path_to_theme(); ?>/css/bootstrap/css/bootstrap.min.css" />
Resource interpreted as Stylesheet but transferred with MIME type
Take Note: Drupal 6 is no longer officially supported by the community. While there are some places that are offering long term support for Drupal 6, you should plan to move to Drupal 7 or 8 soon.
You appear to have two issues: the path to your CSS file and the headers sent by the web server for CSS files. The main issue you mentioned is the CSS path, so most of the information here pertains to that.
path_to_theme() provides a reference that is relative to Drupal root, which is usually the site root (
/) but could be something else if Drupal is running in a sub directory or some other configuration. You need to prepend the result of
base_path() if you are going to hard code the link.
You are nearly always better off using
drupal_add_css() from within a hook in your module or template.php instead of hard-coding the link in a tpl.php file.
Drupal_add_css() allows Drupal to process your CSS through its built-in aggregation. The
hook_preprocess_HOOK() functions are common places to add CSS and JS files.
The MIME type notice is likely caused by the web server not detecting CSS files correctly and setting the right headers. The fix for that depends on both your web server selection and hosting environment (Google should help you find the issue and fix it).