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Diacritics are those dots, dashes, and squiggly lines above and below letters which one sees when Sanskrit, Pāli, and Japanese are transliterated using Roman characters, ie the letters we use to write English. Throughout this website I have opted to include all diacritics for Roman script transliterations of Sanskrit, Pāli, and Japanese. Why? Isn't this just being pedantic? No, it isn't. Diacritics convey important information. It is important for instance to distinguish between "t" and "ṭ" because they are completely different letters of the alphabet, and are pronounced differently. If you look at the table of Siddhaṃ characters you can see that the two letters have nothing in common when written in Siddhaṃ.
It may be argued that English speakers will inevitably pronounce them the same anyway so why bother? There's a story about a person who goes to a Chinese restaurant and orders a steak. The waiter comes along a few minutes after it's delivered to the table and asks how the steak is. The person says "it's rubbery". The waiter says: "Rubbery? Oh that's good, I'rr go and terr the chef, he'rr be so preased to hear it". The Chinese can also poke fun at us: for instance the syllable 'ma' can mean mother, numb, horse, or scold, depending on the tones used and it takes time to master tones when one is not a native speaker.
The point here is not to make fun of other people's pronunciation, but to point out that it makes a difference in English, so why would Sanskrit be different? Precision in pronunciation is desireable because it aids communication, and reduces the chances of being misunderstood. Wrtng Snskrt wtht dcrtcs s bt lk wrtng Nglsh nd mssng t th vwls. It may be possible to reconstruct and understand the original... or it may not.
If you are having problems seeing all of the characters with diacritical marks, then the Unicode website may have some helpful information about configuring your computer or web browser to display them. If you are using the Firefox web browser you shouldn't be having any problems - which is a good reason to switch from Microsoft Internet Explorer.