when you design a webpage using just HTML, CSS & JS, you can use many features in HTML editors, in NetBeans, for example, the ending tag is highlighted when you hover over the starting tag, and such things.
But after converting your webpage to a wordpress theme, every single file breaks into few files (the starting tag of a div may be in the header.php and the ending in footer.php), css and js file addresses change, ... so the editor can not perform well, and not only many features may be disabled, but also it thinks you have some syntax errors in your files, simply because an editor can't run your php code and see the result.
I myself invented a very slow and not so standard method! Firs I create an index.php from the very first index.HTML webpage. Every time I want to edit my theme, I open it and work on that, then break it to the Wordpress theme files.
In my index.php, I use some codes like:
<?php
/*
* Simulating some WP functions!
*/
function bloginfo() {
echo "//localhost/site";
}
?>
And some HTML comments like these:
<!-- WP header START -->
... this part is meant to be copied to header.php
<!-- WP header END -->
...
<!-- WP sidebar START -->
...