When working with websites, you definitely need a plain text editor.
When you edit a file with a word processor like LibreOffice Writer or Microsoft Word, that word processor will add markup; bold, italic, justified, page styling, etc. When someone else opens that very same document, it will be displayed the way you want it.
Each file format – be it .odf, .rtf or .docx – has its own markup rules. These formats are intended for humans.
Other files should not only be readable by humans, but also be understood by software applications like browsers and web servers. These programs do no know how to handle the markup op word processors.
Even more, additional markup might ruin the file and cause your website to malfunction. You do not want that.
A plain text editors is an editor that does not add markup to the file content.
That is why it is advised to use a plain text editor for web related files. This includes stylesheets like the style.css of your theme, PHP files like the WordPress configuration file wp-config.php, but also HTML documents and .htaccess files.
So, do not use a word processor. You even better avoid Windows’ default plain text editor Notepad. Although a plain text editor, this editor has the habit of appending the .txt extension when saving files.
There are more than enough alternates for Notepad specifically developed for editing code files. These editors add nifty features like line numbering, and syntax highlighting.
Sublime Text is definitely my preferred editor. Sublime Text is cross platform, it is available for Windows, Mac and Linux environments.
However, Sublime Text is not free, it has a price tag of $70 for a personal license that allows you to use Sublime Text on all platforms and machines you work on.
You can evaluate Sublime Text for free though. Simply download Sublime Text and start using it.
Of course $70 can be a lot of money, certainly when you will be using the editor only occasionally. Fortunately, there are several free and goo alternatives available too.
Free Text Editors
When you work on Linux, you can turn to gedit. I have yet to encounter a Linux distribution where gedit is not available from the distro’s repository – no matter the graphical user interface.
Another good alternative is Kate, created for the KDE desktop, but also works with other GUIs.
So please do yourself a favor and use a plain text editor for files that require to be machine readable. There are enough good editor available for free, when you can not effort a license.