If you are not familiar with the default generated CSS in WordPress, take a look on the following list. Also the beginners can follow the standardize of the theme development in CSS classes from the listed class which are automatically generated by WordPress.

Default Post Styles

.post-id {}
.post {}
.page {}
.attachment {}
.sticky {}
.hentry {}
.category-misc {}
.category-example {}
.tag-news {}
.tag-wordpress {}
.tag-markup {}

Body Class Styles

.rtl {}
.home {}
.blog {}
.archive {}
.date {}
.search {}
.paged {}
.attachment {}
.error404 {}
.single postid-(id) {}
.attachmentid-(id) {}
.attachment-(mime-type) {}
.author {}
.author-(user_nicename) {}
.category {}
.category-(slug) {}
.tag {}
.tag-(slug) {}
.page-parent {}
.page-child parent-pageid-(id) {}
.page-template page-template-(template file name) {}
.search-results {}
.search-no-results {}
.logged-in {}
.paged-(page number) {}
.single-paged-(page number) {}
.page-paged-(page number) {}
.category-paged-(page number) {}
.tag-paged-(page number) {}
.date-paged-(page number) {}
.author-paged-(page number) {}
.search-paged-(page number) {}

Default Menu Styles

#header .main-menu{} // container class
#header .main-menu ul {} // container class first unordered list
#header .main-menu ul ul {} //unordered list within an unordered list
#header .main-menu li {} // each navigation item
#header .main-menu li a {} // each navigation item anchor
#header .main-menu li ul {} // unordered list if there is drop down items
#header .main-menu li li {} // each drop down navigation item
#header .main-menu li li a {} // each drap down navigation item anchor

.current_page_item{} // Class for Current Page
.current-cat{} // Class for Current Category
.current-menu-item{} // Class for any other current Menu Item
.menu-item-type-taxonomy{} // Class for a Category
.menu-item-type-post_type{} // Class for Pages
.menu-item-type-custom{} // Class for any custom item that you added
.menu-item-home{} // Class for the Home Link

Default WordPress Widget Styles

.widget {}

#searchform {}
.widget_search {}
.screen-reader-text {}

.widget_meta {}
.widget_meta ul {}
.widget_meta ul li {}
.widget_meta ul li a {}

.widget_links {}
.widget_links ul {}
.widget_links ul li {}
.widget_links ul li a {}

.widget_archive {}
.widget_archive ul {}
.widget_archive ul li {} 
.widget_archive ul li a {}
.widget_archive select {}
.widget_archive option {}

.widget_pages {}
.widget_pages ul {}
.widget_pages ul li {}
.widget_pages ul li a {}

.widget_links {}
.widget_links li:after {}
.widget_links li:before {}
.widget_tag_cloud {}
.widget_tag_cloud a {}
.widget_tag_cloud a:after {}
.widget_tag_cloud a:before {}

.widget_calendar {}
#calendar_wrap {}
#calendar_wrap th {}
#calendar_wrap td {}
#wp-calendar tr td {}
#wp-calendar caption {}
#wp-calendar a {}
#wp-calendar #today {}
#wp-calendar #prev {}
#wp-calendar #next {}
#wp-calendar #next a {}
#wp-calendar #prev a {}

.widget_categories {}
.widget_categories ul {}
.widget_categories ul li {} 
.widget_categories ul ul.children {}
.widget_categories a {}
.widget_categories select{}
.widget_categories select#cat {}
.widget_categories select.postform {}
.widget_categories option {}
.widget_categories .level-0 {}
.widget_categories .level-1 {}
.widget_categories .level-2 {}
.widget_categories .level-3 {}

.recentcomments {}
#recentcomments {}
#recentcomments li {}
#recentcomments li a {}
.widget_recent_comments {}

.widget_recent_entries {}
.widget_recent_entries ul {}
.widget_recent_entries ul li {}
.widget_recent_entries ul li a {}

.textwidget {}
.widget_text {}
.textwidget p {}