Overview of the Contribute permission level in SharePoint Server
APPLIES TO: 
2013 2016 2019 Subscription Edition 
SharePoint in Microsoft 365
About updating web files
Web files enable you to implement various customization options for a web page. Web files can contain scripts (for example, JavaScript) that can call web services and interoperate with data on a site. Web files are stored as a modifiable list, based on their file name extensions.
A member of the Farm Administrators group can use Microsoft PowerShell cmdlets to configure web files that have the following file name extensions:
.ascx
.aspx
.asmx
.master
.jar
.swf
.xap
.xsf
.xsn
The tasks that require the Add and Customize Pages permission in SharePoint Server include the following:
Update the content of a web file
Move a web file
Upload a web file
Rename a web file
Publish, migrate, import and export a web file
Users with the default Contribute permission level in SharePoint Server can perform the following tasks:
Check in, check out, or revert a web file
Revert a version from version history for a web file
Delete a web file
Recycle a deleted web file
About editing Web Parts
To add or edit Web Parts that developers have marked as unsafe for editing, SharePoint Server users must have the Add and Customize Pages permission.
By default, the following Web Parts are marked as safe for editing and can be added or edited by users who have the default Contribute permission level:
Basic Chart Web Part
Image Web Part
Page Viewer Web Part
Picture Slideshow Web Part
Relevant Documents Web Part
Site Users Web Part
Title Bar Web Part
User Tasks Web Part