What version of Windows and Windows Notepad are you using? In my experience (and I just confirmed it), Windows 7 Professional Notepad (about notepad: MSWin 7 Pro, v6.1 (7601: SP1)) does not do what you describe:
If I select a portion of my text (as you showed in the screenshot) and select Format > Font > Bold Oblique, the whole document changes to Bold Oblique. Moreover, if I save then exit notepad, then open a new blank notepad document, and start typing, the new document is also in Bold Oblique. If I go to Format > Font > Normal, and open my saved document, the document I saved as Bold Oblique is no longer Bold Oblique. In other words, it behaves exactly as Notepad++'s Style Configurator > Global Styles > Default Style > font selection.
Notepad and Notepad++ are both, at the core, plain-text editors. NPP has the benefit (inherited from it’s core Scintilla) that it can run a syntax lexer, and apply formatting based on that lexing; you can also use other plugins for NPP that allow further highlighting or processing of the text file; these features make it exceedingly useful for coding, and other similar tasks. Plane old notepad can just edit the text – you can define the default presentation of the whole text inside notepad, but it doesn’t change the fact that it’s a plain text file, and all will be rendered the same. (Note: I haven’t used Win8’s or Win10’s Notepad, so they may have added features, but it would surprise me if they saved on-the-fly formatting in the text file and still called themselves Notepad.)
If you want on-the-fly markup in Notepad++, you’ll need to pick one of NPP’s builtin Language syntax highlighters, or create a User Defined Language. I’ve got a simplistic UDL defintion for Markdown; I’ve also seen mention on the forum about TODO syntax highlighters (search the forum for TODO, I think). To copy my UDL definition, save the XML below as a file, then use Notepad++ and Language > Define Your Language > Import, and select the file, and restart NPP. If I then edit a file that ends in .md or .markdown, or if I manually select Language > Markdown, it does a simplistic inline-rendering of the markdown. If I want a more detailed rendering of the Markdown, I use PreviewHTML to render the Markdown into a new pane in NPP. See more details about markdown and the rendering at the following links:
Offical Markdown Syntax Quick Version of my PreviewHTML -> Markdown Long Version @nea’s plugin(I haven’t tried @nea’s plugin, since my setup works okay for me… but NPP doesn’t play well with PreviewHTML – it moves it to the disabled plugins on every upgrade of NPP – so you might want to start with @nea’s plugin instead. I don’t know what all it does – you might not even need my UDL – but I remembered noting that the description sounded useful)
<NotepadPlus> <UserLang name="Markdown" ext="md markdown" udlVersion="2.1"> <Settings> <Global caseIgnored="yes" allowFoldOfComments="no" foldCompact="no" forcePureLC="0" decimalSeparator="0" /> <Prefix Keywords1="yes" Keywords2="yes" Keywords3="yes" Keywords4="yes" Keywords5="yes" Keywords6="no" Keywords7="no" Keywords8="no" /> </Settings> <KeywordLists> <Keywords name="Comments">00# 01 02 03<!-- 04--></Keywords> <Keywords name="Numbers, prefix1"></Keywords> <Keywords name="Numbers, prefix2">#</Keywords> <Keywords name="Numbers, extras1">A B C D E F a b c d e f</Keywords> <Keywords name="Numbers, extras2"></Keywords> <Keywords name="Numbers, suffix1"></Keywords> <Keywords name="Numbers, suffix2"></Keywords> <Keywords name="Numbers, range"></Keywords> <Keywords name="Operators1"></Keywords> <Keywords name="Operators2">* - + ></Keywords> <Keywords name="Folders in code1, open"></Keywords> <Keywords name="Folders in code1, middle"></Keywords> <Keywords name="Folders in code1, close"></Keywords> <Keywords name="Folders in code2, open"></Keywords> <Keywords name="Folders in code2, middle"></Keywords> <Keywords name="Folders in code2, close"></Keywords> <Keywords name="Folders in comment, open"></Keywords> <Keywords name="Folders in comment, middle"></Keywords> <Keywords name="Folders in comment, close"></Keywords> <Keywords name="Keywords1">http:// (http:// https:// (https:// (/ / file:// (file://</Keywords> <Keywords name="Keywords2">==== ----</Keywords> <Keywords name="Keywords3">*** ___</Keywords> <Keywords name="Keywords4">** __</Keywords> <Keywords name="Keywords5">* _</Keywords> <Keywords name="Keywords6"></Keywords> <Keywords name="Keywords7"></Keywords> <Keywords name="Keywords8"></Keywords> <Keywords name="Delimiters">00[ 01 02] 03` 04 05` 06< 07 08> 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23</Keywords> </KeywordLists> <Styles> <WordsStyle name="DEFAULT" fgColor="333333" bgColor="FFFFFF" fontName="" fontStyle="0" nesting="0" /> <WordsStyle name="COMMENTS" fgColor="008000" bgColor="FFFFFF" fontName="" fontStyle="0" nesting="0" /> <WordsStyle name="LINE COMMENTS" fgColor="FF8040" bgColor="FFFFFF" fontName="" fontStyle="1" nesting="67108864" /> <WordsStyle name="NUMBERS" fgColor="0000FF" bgColor="FFFFFF" fontName="" fontStyle="0" nesting="0" /> <WordsStyle name="KEYWORDS1" fgColor="408080" bgColor="FFFFFF" fontName="" fontStyle="4" nesting="0" /> <WordsStyle name="KEYWORDS2" fgColor="FF8080" bgColor="FFFFFF" fontName="" fontStyle="1" nesting="0" /> <WordsStyle name="KEYWORDS3" fgColor="000080" bgColor="FFFFA0" fontName="" fontStyle="3" nesting="0" /> <WordsStyle name="KEYWORDS4" fgColor="400080" bgColor="FFFFFF" fontName="" fontStyle="1" nesting="0" /> <WordsStyle name="KEYWORDS5" fgColor="000080" bgColor="FFFFFF" fontName="" fontStyle="2" nesting="0" /> <WordsStyle name="KEYWORDS6" fgColor="000000" bgColor="FFFFFF" fontName="" fontStyle="0" nesting="0" /> <WordsStyle name="KEYWORDS7" fgColor="000000" bgColor="FFFFFF" fontName="" fontStyle="0" nesting="0" /> <WordsStyle name="KEYWORDS8" fgColor="000000" bgColor="FFFFFF" fontName="" fontStyle="0" nesting="0" /> <WordsStyle name="OPERATORS" fgColor="0000FF" bgColor="FFFF00" fontName="" fontStyle="1" nesting="0" /> <WordsStyle name="FOLDER IN CODE1" fgColor="000000" bgColor="FFFFFF" fontName="" fontStyle="0" nesting="0" /> <WordsStyle name="FOLDER IN CODE2" fgColor="000000" bgColor="FFFFFF" fontName="" fontStyle="0" nesting="0" /> <WordsStyle name="FOLDER IN COMMENT" fgColor="000000" bgColor="FFFFFF" fontName="" fontStyle="0" nesting="0" /> <WordsStyle name="DELIMITERS1" fgColor="408080" bgColor="FFFFFF" fontName="" fontStyle="1" nesting="0" /> <WordsStyle name="DELIMITERS2" fgColor="000000" bgColor="CCFFFF" fontName="" fontStyle="0" nesting="0" /> <WordsStyle name="DELIMITERS3" fgColor="008000" bgColor="FFFFFF" fontName="" fontStyle="1" nesting="0" /> <WordsStyle name="DELIMITERS4" fgColor="FF8000" bgColor="FFFFFF" fontName="" fontStyle="0" nesting="0" /> <WordsStyle name="DELIMITERS5" fgColor="000000" bgColor="FFFFFF" fontName="" fontStyle="0" nesting="0" /> <WordsStyle name="DELIMITERS6" fgColor="000000" bgColor="FFFFFF" fontName="" fontStyle="0" nesting="0" /> <WordsStyle name="DELIMITERS7" fgColor="000000" bgColor="FFFFFF" fontName="" fontStyle="0" nesting="0" /> <WordsStyle name="DELIMITERS8" fgColor="000000" bgColor="FFFFFF" fontName="" fontStyle="0" nesting="0" /> </Styles> </UserLang> </NotepadPlus>