How to hide SharePoint top bar (header) and left navigation from SharePoint custom master page using CSS?

You can either create a blank master page using SharePoint designer or

use the following styles:

#s4-ribbonrow, .ms-cui-topBar2, .s4-notdlg, .s4-pr s4-ribbonrowhidetitle,
.s4-notdlg noindex, #ms-cui-ribbonTopBars, #s4-titlerow,
#s4-pr s4-notdlg s4-titlerowhidetitle, #s4-leftpanel-content,#sideNavBox
{
    display:none !important;
}

.s4-ca
{
    margin-left:0px !important; margin-right:0px !important;
}

#contentBox
{
    margin-left:0px;
    margin-right:0px;
    min-width:auto;
}

#contentRow
{
padding-top:0px !important;
}

#s4-workspace
{
 width:auto !important;
}

#s4-bodyContainer
{
 padding:0px;
}