Move your template folder right under resources:

src/main/resources/static/css (for CSS files); src/main/resources/templates (for HTML templates). 

Then correct the link tag as follows (relative or absolute):

<link href="../css/firstcss.css" rel="stylesheet"> <link href="/css/secondcss.css" rel="stylesheet"> 

The old solution with static in front doesn't work for me.

Move your template folder right under resources:

src/main/resources/static/css (for CSS files); src/main/resources/templates (for HTML templates). 

Then correct the link tag as follows (relative or absolute):

<link href="../css/firstcss.css" rel="stylesheet"> <link href="/css/secondcss.css" rel="stylesheet"> 

The old solution with static in front doesn't work for me.