Currently, there is a lack of truly useful standards for cartographic styling in webservices. The OGC SE-SLD [1] standard dates back to 2006. It was originally designed for Web Mapping Services only, and is therefore not well-suited for modern web-based applications. It is also technically out-dated, as it is non-extensible and only offers an XML encoding. Although it is used frequently in 'traditional' GIS & Geowebservices, most software and systems only partly implement the standard (e.g. QGIS & Mapserver), and the demands of the system sometimes require non-standard extensions (e.g. Geoserver extends SLD in 'vendor-specific' ways). Currently, the OGC SLD-SE working group is proposing a major redesign [2], but progress is slow and limited in scope.

In the past years, several alternatives to styling web-cartography have been proposed and used, based on modern syntax frameworks such as CSS (e.g. CartoCSS [3]) or JSON (e.g. Mapbox GL Style Specification [4]). These types of styling descriptors have pro's and con's. They mostly are linked to specific software. They tend to come and go in fairly short cycles, and no new standard has emerged.

For this research, you should get a thorough overview of the established as well as the emerging mechanisms for cartographic styling in webservices. The advantages and disadvantages of the various solutions should be compared in terms of "cartographic strengths & weaknesses", i.e. describe how well each solution supports creating good maps in web applications. You will have to look into cartographic theory as well as into the practical technology involved in web-mapping, to be able to come up with a useful and scientifically sound system of comparison. Your findings should then be used to give directions for the future standardisation of cartographic styling.