Writing came to Insular Southeast Asia in the 8th century, brought by traders from the ancient Indian kingdom of Pallava. The Pallava script, a variant of the Grantha, was adopted by the local Javanese to write their language. While at first the script completely resembled Pallava, soon it evolved into a distinctive form called the Kawi script.
Geographically, Kawi was found primarily on Java, Bali, and southern Sumatra, but a few inscriptions have been found as far as the Philippines. This points to a more widespread use of this script in Southeast Asia, but most likely written or carved on perishable materials that have not survived to the modern day.
The Kawi script is a typical Brahmi-style syllabic alphabet where every letter represents a syllable rather than a simple sound. Three letters are used for the sounds /a/, /i/, and /u/, but only at the beginning of a word. The rest of the letters denote syllables, each with a starting consonant and the "inherent" vowel of /a/.
The following chart is the basic Kawi script.
In addition, there are a set of diacritical marks that are placed around letters to modify their phonetic value. Most of these marks are used to change the inherent vowel from /a/ to another vowel or dipthong. Two diacritical marks denote the endings /-ang/ and /-ah/. One mark actually removes the vowel completely, leaving only the consonant part of the letter.
Even though there is a mark to suppress the inherent vowel, consonant clusters are actually represented by placing two letters stacked on top of each other. The first consonant in the cluster would be the letter on the top but with its inherent vowel silenced, and the second consonant would be the letter on the bottom. For most part the regular letter forms are used on both the top and the bottom, but for the letters /ya/ and /ra/ special forms are used instead, as illustrated in the following example.
By the 13th century, the form of the Kawi script has evolved into its direct descendent, Javanese. In fact, Kawi is sometimes called Old Javanese. But in addition, the Kawi script likely gave rise to many later scripts of Indonesia, Malaysia, and Philipines such as Rejang, Buginese, Makasarese, Tagalog, Mangyan, Batak, and many others. Hence the Kawi script can be considered the progenitor of writing in Insular Southeast Asia.